Lemmy instances are kinda like islands, but you can visit and see other islands that are on good terms, or "federated." Federation can be one-way, ie you can see and comment on another instance's posts but they can't see yours, or it can be two-way, and you can comment back and forth. You are on Lemmy.world's view of a Lemmy.ml post. There are comments from Hexbear and Lemmygrad users on this post that I can see, but you can't, like this one.

Lemmy.world is defederated from Hexbear.net and Lemmygrad.ml, the two biggest communist instances. In order to see their content and interact with their users, you need an account on an instance like those two, or Lemmy.ml, Lemmy.zip, etc. You don't need one for each instance, just one federated with what you want to see.

Does that make sense?