From my limited knowledge, it’s a subpart of the Fediverse which follow the design philosophy of threads, like Reddit. So basically Lemmy, Piefed, Mbin (i think Kbin is dead) and apparently now Catodon. Also it could technically include Threads, which has or plans to have the ActivityPub support, but since it’s operated by Meta, i’ve seen a lot of opposition to it being included, as well as people criticizing the “Threadiverse” terms for being too close to the Threads app name.