A federation is just a group of entities that has agreed to work together.

De-federating means that you don’t want an entity in the group so you refuse to work with them.

For the threads situation. Say all of the nerdy kid’s in high school decided to make a club (current fediverse) and one of the popular kids (meta) wants to join, but the existing nerdy kids in the club feel that the popular kid is really only joining to take over and ruin their club. Many of the nerdy kids will ignore the popular kid to try to freeze him out of the group (defederate).

Some existing members try to block, some will accept, and some will wait and see what happens. In all cases the popular kid has already disrupted and created divisions in the original club members.

To put this a different way, it functions like a block. People want to block the entirety of Threads because we don’t trust Threads.

@Roflmasterbigpimp Every account on the Fediverse is hosted by an instance on a server, the admin of each instance can choose to defederate from any other instance if they don’t want content or users from that instance to be intractable with their users or visa versa. This is often done due to differences in “terms of service”, for things like hate speech, CP, or just political differences. Some instances will have zero NSFW policies, some will have defederated communities/magazines for content private to that instance (lemmynsfw does thus for example).