Can someone explain to me how to use the Feidverse?

Each Instance / Server is able to use the data of the Activity Pub Protocol. If you see comments on Lemmy where people use hashtags or mention Users with an "@", they are probably answering through Mastodon.
- 😂
  
  It's pretty old. Feddit.de was replaced with feddit.org a while ago.
- Thanks dude.
- With just a single account on ~~any~~ most Lemmy server(s) you can read and write ~~everything~~ most things everywhere else.
  Click on the "all" button and you'll see content from ~~all~~ some subreddits on ~~all~~ some servers.
  I know I am nit-picking, but this over-selling of federation as if it "just works" and takes no manual effort almost drove me away when I first joined. It did and still does drive me away from Mastodon, but Lemmy is not as problematic.
  At any rate, I know we don't want to overload new users with talks of defederation and unfederated content, but pretending it doesn't exist makes for a very frustrating time the first time someone realises they can't see 50% of the posts or thinks Lemmy as a whole is really quiet when it's because their server isn't federated 100% with every other server.
- I'm also pretty new around here and I understand the connection between the lemmy instances but how does mastodon fit into this? Can I comment here with my mastodon account and vice versa?
  
  You can follow Lemmy communities on Mastodon but that will cause every single comment to appear in your feed, so depending on how much activity they get, they will probably flood your feed. You can post to Lemmy communities from Mastodon by mentioning them, to the best of my knowledge. In either case you can reply to whatever you see on Mastodon and it will appear as a Lemmy comment.
  You cannot follow Mastodon accounts on Lemmy.

Imagine if instead of just reddit.com there was also reddit.org and reddit.co.uk and reddit.nl and reddit.social and (etc etc) all on a different server from each other and each with its own set of users and subreddits. But each user and each subreddit could be viewed and joined by any user from any server - that's Lemmy.
So you're on the piefed.social server (but on the fediverse servers are called 'instances') and I'm on the lemmy.blahaj.zone instance but we can both see, subscribe to and post to a community (the Lemmy name for subreddits) on an instance neither of us are members of - the asklemmy community on the lemmy.world instance.
Take a look at your screen (or app if you're on mobile) and you'll see 'Local', 'Subscribed' and 'All'. If you select 'Local' you will see a list of posts from Communities that are on your home instance (which is piefed.social in your case). If you selected 'Subscribed' you'd see posts from all the Communities you chose to join/subscribe to across all instances. If you choose 'All' you'll see posts from the entirety of Lemmy whether you subscribed to them or not. Whichever view you choose can be sorted by things like 'new', 'active', 'hot' etc.
To find Communities you're interested in joining, use the Search function, type in a keyword and select 'Communities'.
- This is mostly correct, but another point to consider (once OP becomes more familiar with the basics) is the concept of defederation. Basically, not all instances permit interactions with each other. Many smaller instances have defederated from lemmy.world, for example. That means that users on those instances can not directly interact with communities hosted on lemmy.world, although you could still encounter users from that instance in a post on a community in a different instance that is federated with both.
  If the instance you are currently on has defederated from another one that you would like to browse, you will need to create a new user account on that instance (or one that hasn't defederated from it).
  
  Many smaller instances have defederated from lemmy.world, for example
  Why?
- I'm using Lemmy Explorer to search for communities. When I find them it says "Subscribe from Remote Instance: Enter the instance you would like to follow this community from." I type in piefed.social because that's what I'm on, but then it gives me a black screen and a not found notice. Most of these are lemmy.world communities so I should be able to subscribe to them, no? Or am I typing in the wrong thing?
  
  You could try a Lemmy app that gives you a more reddit-like experience. There are several. I use Sync, but only because it's what I'm familiar with from the reddit days. There are plenty of alternative (and often better) options. I can find communities from instances my instance federates with in the search bar, and I even have several different accounts in different instances (world, beehaw, hexbear, etc) to see content my instance doesn't federate with which I can switch to with a dropdown menu easily.
  Edit: I assume it's been explained, but not every server/instance talks to each other instance, called federation. This is often for political, spam, or harassment reasons. It may be worth using alternative accounts or using Lemmy from another instance logged out if viewing content from those communities you don't federate with is important to you. The big instances federate with mostly everyone, but there are always exceptions.
  
  I'm not 100% sure what Lemmy Explorer is....but assuming its a tool to help you find communities?
  If thats so, just make a note of the community address, switch back to your logged in account and search for that community whilst logged in to your piefed.social account
  I know this all feels disorientating and needlessly confusing but its worth it. Stick with it and keep asking for help when needed.

Another thing that might be a bit different from reddit, is that lots of folks here recommend using the Block options for users / communities. Lemmy is a smaller place, so if a particular user keeps posting stuff that isn't what you want to see in a community it can really ruin the experience. It's totally okay to just block people, and is the generally recommended way of dealing with trolls.
Federation, multiple instances, among other things, means that lemmy as whole is less strictly governed by mods than reddit. That's good in many ways, but it does mean that we users have to take some responsibility to create the lemmy we want, and that sometimes means blocking people.

When I'm on my home instance (piefed.social for you I suppose), I see a list of subscribed communities on the sidebar. I think you can go into your profile and see as well. I used to just look at someplace like https://lemmyverse.net/communities to find new communities, but I don't know what the goto is now, it's been awhile since I've looked.
One weakness/strength of the fediverse is that it's not a single point of failure. By that I mean reddit might just nuke /r/foobar and it's all gone. The fediverse may break up that single community into several, ie foobar@piefed.social, foobar@lemmy.world, foobar@whateverhost. Now you have 3 or more communities discussing foobar, but they aren't linked. The weakness is that they aren't linked and can split the userbase. The strength is that if the owner of lemmy.world fucks off and their server disappeared, the other choices are still there. If you find
<hobby or interest>
@skism.net, but there only are a handful of people and 3 posts over the past 6 months, you may find the same
<hobby or interest>
@another.com that has a super active userbase.
It can be a little annoying at first, if you are used to reddit and 10k people commenting stupid shit all over the place, most of Lemmy you'll usually find 10s or 100s of people commenting stupid shit. It's worth it for me, and I haven't opened reddit in over a year. Your mileage may vary.

I'm not sure how common this knowledge is, but the default Lemmy web-interface sucks. There are web portals that improve it considerably. Some of them may be available direct on your instance. If not, I use https://alexandrite.app/ for my frontend.
There are others, at least one of which looks like old.reddit, but I'm not sure the address of them. I would trigger Cunningham's Law, but I don't want to add confusion.

Well for starters this isn't Reddit so you can edit that typo in your title 😉

You seem to be doing just fine

Use the search function. It had more options, but you can search for communities in servers with which your instance is federated
The biggest issue you will encounter is parallel evolution. If you search for "linux" communities, you'll find a dozen of them, all named "linux", each on a different server. The naïve, easy way to choose is to look for the communities with the highest member count - but this is not always the best way to find the one you want to join. !linux@pentakoala.net will be full of people who bitch constantly about software that uses licenses other than GPLv3 and get downright nasty about people using MIT. Totally hypothetical, but you get my point - you have to peek into the communities and read some posts to differentiate their tone.
Browse "All" occasionally. Sort by "New." Yeah, you'll get a lot of furry porn, and a fair amount of OnlyFans fishing posts, but you'll also find !superbowl@lemmy.world!

Voyager for iOS is a pretty great client!
- I use Voyager as well (on android), can confirm it's excellent! :) The UI is very similar to Reddit's.

I recommend using the Voyager mobile app, it makes it very easy to see your communities, browse all, etc
- Piefed isn't lemmy so voyager won't work. I believe interstellar does work (I use it for my alt on mbin and it's a nice app)
  OP, as you see, you're not stuck in your instance and can interact directly with people/communities from other instances. I believe that piefed also let you follow Mastodon account, but I haven't tried piefed yet

Idk how piefed is but on lemm.ee I'm able to search for communities across all instances lemm.ee is federated with. I found most of mine by just searching either the names from Reddit or something more generic about the topic.

Looks like on top of your website is the "topics" and on the drop down you can go to the "all" https://piefed.social/communities and then you can join other sites communities etc. Surprised nobody really even answered this lol
and on the main page you have local / popular / all for feeds you can view too. It should have a "subscribed" so you can see the ones you are subscribed too, I don't have an account though so can't see

Tbh idk what Piefed.social is but I see your comment from Boost. I think that explains the fediverse? 😅
As far as communities work, idk if you click this link, do you see memes from within your app (or does it open your browser)? !memes@lemmy.ml