I'm going off of memory here and did not do any research at all, so expect errors:

Hexbear is based on Lemmy, a reddit clone whose cool special feature is supposed to be federation, which I'll explain at the end. Early on in the site's history, the devs decided to use a forked version of the code so they could change it to handle things we wanted like pronouns, and also rewrite the frontend in React for some reason. All the changes meant Hexbear wasn't able to federate anymore.

After three years of Lemmy getting better while Hexbear's devs were a constantly changing group of people without enough time who spent what little time they had putting out fires, being on a forked version stopped making much sense. So the developers decided to move everything back to a normal Lemmy (I don't know how much customization we have now, presumably still some). Moving all of that around was why the site was down yesterday.

Now Hexbear is able to federate, but it has not yet done that. We are not federated. You can't do federation stuff right now. I assume the devs are going to wait for things to be slightly less on fire before they try.

Once Hexbear federates, there will be a specific set of other Lemmy websites (starting with Lemmygrad.ml only), where you can find and subscribe to their coms, from Hexbear, if you go to the communities tab and go out of your way to find them. Additionally, commentators from both sites will be able to comment on posts from the other site.