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.

In fact, the fediverse is divised in two part :

• microblog (mastodon, firefish, castodon...)
• threadiverse (lemmy, mbin, piefed, nodebb...)

Microblog users can interact with the threadiverse. They can subcribe to us and our community. However, our post on the threadiverse, in microblog won't be structured, it is messy.

Threadiverse user can't follows microblog users nor see microblog user post except when they post in our community. Last point, the largest userbase are microblog users so i believe we should reach them and crosspost their content there. :3

Aaaaah, j'espere que non 💀