I didn’t research it too in depth, but from the quick understanding I have about it, it works something like this:
Fediverse is just a name for servers that implement the protocol ActivityPub. The protocol defines a way how different servers should share data and interact with eachother.
For example, you have Mastodon, which has decided to implent a Twitter-like social network. Or Lemmy, that implemented Reddit-like social network, or the Pixelwhatever, that implements a picture sharing service.
When someone hosts an instance of the social network, you use it just like you would a normal social network. But in addition to that, ActivityPub protocol defines a way how can the servers talk to eachother - how they share data. So a user of i.e lemmy.ml can ask their home instance “can you let me subscribe to posts from programming.dev community?”, and the instance will use the protocol to start fetching posts from there, to show to the user, while also sending back comments the user makes on said treads.
It would also be possible to connect Mastodon with Lemmy - assuming that some kind of UI is created that would handle the difference in type of content. But the ActivityPub protocol gives them a common language about how to interact. So if a Mastodon creates a special post type for Lemmy posts, users can start requesting “Hey, can you get me posts from this Lemmy instance?”, and it will be able to get them though ActivityPub and then somehow show it to them.
A fediverse is just a network of servers that are allowed to talk to eachother. If Meta would join the Fediverse and we did not defederate with it, it would mean that Meta’s servers can ask any instance to give them their content and let their users interact with it - but Meta will decide how is it shown. So Meta can riddle it with ads, run analytics about how are users interacting with the posts, scrape it for user data or simply use the content we have as a way how to kickstart their platform that noone would want to use in the first place.
So, defederation just means “I don’t want to talk to you, and I will not give you any data.”