To further what rideranton said, the way federation works is by mirroring content across different websites. You’re interacting with copies of posts and comments that have been sent to lemmy.world, and when you reply to someone, or post in a remote community, all of that is happening locally. You’re exclusively engaging with the local copy of everything.

When sites are federated, they then sync up their content. If they defederate, that syncing merely stops occurring.

Let’s say there are five websites all federating, A, B, C, D, and E. There’s a popular community, Junk, on B that people on all 5 websites use. This means all 5 sites house a local copy of Junk, and whenever someone posts to Junk their local site pushes those posts out to the other 4.

But one day, B defederates from A, because they decide that there are too many assholes on A. Now, anything people on B post to Junk just never reach A, and similarly anything people on B post to Junk never gets sent to A. But A is still communicating with C, D, and E. Their updates still flow out to those 3 other websites, and updates form them continue to flow back. Similarly, B is still in communication with C, D, and E, so they stay in sync as well. And really, from sites C, D, and E, it kind of looks like nothing happened (though if people pay close enough attention, they may notice that people from A never comment on posts from people using B, and vice versa).

Now, users on A may still be able to see posts and comments from users on B if they arrive on A via C, D, or E, but it won’t work the other way: Because site B has explicitly blocked traffic from site A, any content whose author is on site A will be filtered out when syncing with C, D, or E.