(oversimplified) they are “interconnected” just like emails are interconnected. I can log into Gmail and send an email to someone else at Hotmail. But I can’t actually use Gmail credentials to log into Hotmail.

In the 'verse, “Federation” means every instance (I’m on aussie.zone) holds a copy of every other instance (I can see your post without ever visiting lemm.ee). This reply I’m posting is held in my instance, and a copy is sent to yours.

If lemm.ee shuts down tomorrow, this thread will remain to exist in the thousands of other instances that are federated with lemm.ee today.