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.

Federated = Email. Different email providers (Gmail, Microsoft, iCloud, etc.) can all communicate with each other freely and using the same protocol.

Defederated = No longer federated.

Not federated = Your company’s internal Teams/Slack/whatever messaging system. Even tho they theoretically could communicate without other messengers and/or people outside of the company, they typically don’t do they’re just their own little bubble.

Lemmy is federated, Facebook is not.

Imagine you don't like your neighbor so you build a wall between you and them so you can't see each other or interact.

That's defederation.

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.

Defederation = the defederating server refuses to talk to the other server.

Imagine Hotmail and Gmail. Hotmail decides to defederate from Gmail. Going forward, any Gmail user trying to send mail to Hotmail will simply be ignored, and Hotmail will ignore any email bound for Gmail.

@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).