ActivityPub is the underlying tech that allows for this federated universe (fediverse) of connected servers talk to each other so posts can be seen from other servers. Mastodon is built to use this just like how Lemmy, kbin, PeerTube, misskey, and others are.

As long as two servers are “federated” with each other (open to talking to each other and see each other) then users can see posts from each other’s community. This is where the issue with Threads begins. Fediverse users suspect that Meta is using the fediverse as a kind of data-collection-Trojan horse since it plans on having an ActivityPub interface. Besides that, Threads users will likely bring more toxic commentary with them.

That’s the gist of what I’m getting from other community members. If anyone can put it better then please feel free to correct me or add on/clarify.