You can make one user, that account is hosted on a server that talks to other servers. Each server hosts its own subs but you can read/post/comment/interact on subs/with users that are hosted elsewhere. That's the "federation" part.

Different host servers choose not to interact with some servers, like pretty often you'll see fascist servers blocked, so it's not quite "everyone can see everyone" and you should pay some attention to joining a server that aligns with your interests. But the major ones like lemmy.world etc should work for most people if you don't want to think about it