You created an account on the lemmy.world instance, but there are a ton of other instances out there.

Defederation explanation as far as I understand the topic: By default all instances talk with each other and publish their communities (The federation part). But if an instance decides to defederate from another, it will block the second instance from getting interacting with communities on the first instance. So the users of instance 2 can't post on instance 1.