It’s a progressive political community, based on a fork of lemmy. They added a bunch of features unique to the fork that means that it’s no longer compatible with lemmy. However, hexbear is something like 10 times the size of the largest lemmy instance, and probably has more users than the rest of lemmy combined.

They’re also working at making their fork compatible with lemmy again, so ultimately, they will start federating with lemmy and the rest of the fediverse