I suppose you are on lemmy. Lemmy is kind of the reddit of the fediverse. Imagine that now everyone can create their own reddit server with blackjack and hookers. That is the server you are on, someone decided to create a server for the world. But now I am on another server with different owners, rules and mods (basically blackjack and different hookers). I can interact with you on your server and you can interact with me on my server because our servers are federated.
Now imagine that you can do this for every social media. Reddit, X(itter), Instagram, YouTube, Tiktok, WhatsApp. Everything is available in the fediverse (with more or less issues depending on what you are looking for).
@docker@oakward@feddit.org I did not answer you from mastodon. I was answering you from another instance of lemmy (feddit.org). But now I am writing to you from mastodon (mstdn.social). I don't find this maston-lemmy connection very useful though.
And here I am reading your (Mastodon) comment in a threaded view on my Lemmy app on lemmy.blahaj.zone. I'm curious how my reply appears to you (if it @'s you or not, or how you know it's a reply?)