Imagine that all of the instances existed first - before federation even existed.

So imagine dozens of little, independent message boards - this person created one at this address, and this other person created this other one at this other address and so on.

Okay - then imagine that federation came along, so then all of those individual bosrds started sharing content with all the other boards.

So you were on this one board called lemmy.world. You never moved - you still have the same account at the same board. But now you also see, for instance, content from some other board called lemmy.ml.

Now here's the important bit - you aren't actually looking at lemmy.ml. You're looking at a mirror of it - literally, you're looking at lemmy.ml@lemmy.world. So like this community we're on right now - neither one of us is actually on AskLemmy@lemmy.ml. You're on AskLemmy@lemmy.ml@lemmy.world and I'm on AskLemmy@lemmy.ml@sopuli.xyz. But every few seconds, all three of those actually separate communities sync up with each other, so it acts as if it's just one community.

Does that help?