what is lemmy basically ?

It's like reddit but spread out.
If reddit is one big city, lemmy is dozens of small towns with a good network of public transport between them.

An easy analogy that common users can understand is e-mail. E-Mail is also decentralized, everyone has an e-mail address but everyone uses a different e-mail host (the domain name after the "@"). So e.g. "john.doe@gmail.com" has an account at gmail.com but "jane.doe@mailbox.org" has an account at mailbox.org. Both are completely different, yet they can communicate with each other. There's not one company controlling or storing every single e-mail account or inbox. It's spread out and everyone can choose the mail provider they like or trust the most.
Then you use that as a bridge to explain Lemmy, or Mastodon, or other Fediverse social media platforms. And remind the listener that single companies having full control over everyone's accounts is generally bad and opens the door for all sorts of abuse and manipulation or arbitrariness.

From a user experience its a social media site, like reddit.
And an ELI5 for the technical parts:
It is decentralized which means that no single company owns the whole thing. Anyone can set up a server.
it is also federated which means that servers can communicate with each other. I am able to see your post even though my server is programming.dev, your server is floss.social, and you posted on lemmy.ml.