All the Lemmy (and Kbin) servers (instances) can theoretically talk to each other. You can follow communities on other instances and comment and post like you are used to.

You could think of each server as an independent city-state, each with it’s own leaders, customs and citizens.

If two instances have a feud that can’t be resolved, they will “defederate” and then their users will no longer be able to contact each other.

There are Nazi instances here in the Fediverse (for example), but they get blocked on sight so you almost never see their garbage in the feeds.

Does that help?