Think of email in how accounts are named. Mine is CaptainBasculin@lemmy.bascul.in, where lemmy.bascul.in is the instance I use, similar to an email server.

Now, on email you can send mail to users from other mail servers like from hotmail to gmail, right? It works the same way on lemmy, and also the fediverse. You can see posts made on other instances, reply on them and upvote them.