You have a user account "Got_Bent", on an instance (you can think of this as a "server"), lemmy.world. That's your home instance. Thus, you are @Got_Bent@lemmy.world.

You can view communities on that instance. This post, in fact, is on a community on the lemmy.world instance, !fediverse@lemmy.world.

You can also view communities on any other other instances that lemmy.world is federated with (which is most of them). For example, !unitedkingdom@feddit.uk. By-and-large, you can use them the same way you can communities on your home instance.

Reddit is pretty similar, just that with Reddit, there's only one "instance", Reddit.

Instances might go down (so users with that instance as their home instance can't log in and communities on that instance aren't accessible. Some have certain rules about what users who use that instance as their home instance can do. Others have certain rules about what communities on their instance are allowed to do. For example, my home instance, lemmy.today, wants to avoid defederating with other instances (which means that people with that home instance can see all other content). Some instances, like beehaw.org, want to keep some content that might be objectionable to their users out, and will tend to defederate with other instances if they consider them to be problematic. Some instances allow hosting communities that have pornography (like lemmynsfw.com) and some do not (like sh.itjust.works). Same thing for communities dealing with religion or extreme political views, and so on.

In general, it's helpful to have a home instance in the same rough part of the world as you, as it'll make things more-responsive.