It's the collection of various softwares/sites/platforms that are all able to interact due to operating on the same underlying technology.
Lemmy is part of the Fediverse. Mastodon is also and users there can follow and post in Lemmy communities despite being a totally different thing. Kbin is another Reddit-like, it and Lemmy coexist pretty seamlessly.
And there are so so many more. Fediverse alternatives to FB, Insta, YT, Linktree, etc etc etc.
You'll also hear "Threadiverse" which is what people have taken to calling that part of the Fediverse that focuses on forum thread type discussions (ie Lemmy and Kbin mainly).