In practice, it's Lemmy and Kbin.

More theoretically, it's the part of the Fediverse that deals with threads rather than posts: You share some sort of content (text, images, a video, a link) along with a title, and people comment on it. The most common content type in the Fedverse is posts/microblogs, which it what Mastodon operates with. These posts are generally not visible from Lemmy, meaning that the majority of the Fediverse is invisible from Lemmy; what you see from there is only the "threadiverse" part. Kbin seeks to bridge the two.

You can view threads in Mastodon, but they appear only as the text of the title along with a link to the rest of the content. People can of course comment as normal.