Formerly chapo.chat and r/chapotraphouse, they are a tankie instance that frequently brigades threads that mention them, and engage in harassment campaign against anyone who disagrees with them. They also frequently praise and defend Russia and China, two genocidal dictatorships in which one is engaging in an illegal war against ukraine and china has rampant human rights abuses. Since there are good faith users on that instance, they are more of a hesitation than a censure.
I’ve been seeing a lot of people confusing brigading with being downvoted/disagreed with heavily. Brigading would be if someone from Hexbear specifically posted a link to your post on one of their communities, encouraging others to visit your post, downvote, and harrass you. Hexbear users finding your post while browsing all and dogpiling on you is not brigading, it’s just how it works when there are lots of Hexbear users browsing all new posts.