Lemmy and Kbin are the "threadiverse", where content is submitted by users to specific groups and replies are shown in a tree-structure. In ActivityPub behind the scenes, I believe this is achieved by making non-user Actors that re-post everything @'d at them.
Mastodon is not thread based, users post as themselves and other users follow them directly. Then everything is shown lineally.
The term has actually been rolling around for a few weeks, it's just used to describe Lemmy and Kbin, since they're based around threads instead of twitter-like posts. Nothing to do with Threads by Meta