Each Lemmy intance is hosted by a different person or entity. Lemmy World is hosted by User A, while Beehaw is hosted by User B.

The way federation works, however, is that it allows for User A to connect their server/Lemmy instance to User B's server/Lemmy instance. This allows for users on Server A to talk to people who are using Lemmy over on Server B.

However, letting your servers interact with other servers is not required. If users of one server are causing problems with your Lemmy instance, you can remove your server from theirs, which removes their access. This is referred to as defederation.

So when Beehaw is defederating other servers, it means they are blocking other servers from interacting with Beehaw.

This was a pretty good explanation.

Imagine you are at school and every class display their drawings/paintings. And because the teacher are super nice, they allow you to write comments under the drawings. That school is one lemmy server, with each class being a subreddit.

What is nice about lemmy is that it is federated. This means that by going into your school, you can also view what other schools are doing, and that is really nice to learn what other people are doing. This is also used so that you can have lots of small school everywhere with a smaller size, closer to everyone's home and can also be better for some people.

However, at some point the administrator of beehaw saw that some pupils from lemmy.world were a bit annoying, putting graffitis and making a mess. And the administrator of lemmy.world wanted to have as many pupils in his school as possible, which makes filtering and managing bad pupils difficult. So the administrator from beehaw basically established a rule: the pupils from lemmy.world cam still view our drawings, but they are not allowed to write anything. And this will be the rule until the behaviour of the lemmy.world school improves.