So right now you are on a specific instance of lemmy or Kbin or whatever your preferred fediverse system is. Lemmy.world is an instance. Beehaw.org is a lemmy instance. Users can create accounts on both and can browse the content of the other instances, unless the instance they are on (let’s say 1) defederates from the other (2). Then you cant browse the other instance(2) from within the one (1) that chose to defederate. The content from 2 doesn’t appear anymore on 1. But there’s nothing stopping you from browsing either.