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.