"you know how you can't talk to someone on Twitter.com from facebook.com? But you can email from your @gmail.com to someone with an @yahoo.com address?
That's the difference, federated social media is like email in this way."
I'm mostly sure even my elderly parents understood it when I said it...
And you know how, when you subscribe to a mailing list, you will only receive new mail sent to the list if your server happens to “federate” with the sender’s server?
I’m not talking about blocking, but about being unable to see all replies to a post unless you open it on its home instance, which happens all the time on Mastodon.