Everyone is part of the server they signed up with. Federation allows everyone to see each other’s posts, filtered through your own server.
If another server defederates yours, their users essentially can’t see your (new) comments/posts. But you can see most of theirs.
It is not necessarily a 2 way street. Continuing the example, your server can choose whether or not to defederate back. If it does defederate, you’ll no longer see any new comments/posts from the other server.