I’ve been trying to think of a good metaphor, but I haven’t landed on one yet. The best thing I’ve thought of is the old saying:

Whereever you are, you’re here.

Federation (for lemmy) is the concept of the different instances sharing sublemmys, posts, and comments with each other. So you can be on any instance and interact or subscribe with things from another.

The strong caveat is: some instances turn off federation with others. For example, Beehaw has stronger moderation, and they had problems with users and spam from a specific other instance (lemmy.world was one, I think). So in that case, Beehaw turned off federation with lemmy.world. That means that if you were logged in to Beehaw, you would not see any new content from lemmy.world until they turned federation back on.

edit: I thought of one more thing. For communities that are run by their developers, like Minecraft, Lemmy is a great solution. They could host their own Lemmy instance (lemmy.minecraft) and lock down so that only their sublemmy - /c/Minecraft - is created. But when they federate, they get all the other content from other Lemmy instances.

• As a user, you could sign up with lemmy.minecraft or lemm.ee, etc., and still see everything you want and sub to the /c/Minecraft.

• As mods/admins, they only need to focus on their Minecraft thing. And they have complete control over that, because they can literally shut down the entire instance.