It means “brigading”, specifically “brigading by shitpost”, basically “bullying”. They use the term themselves for their own behaviour on communities outside of their own instance here on lemmy, and it is very typical of their authoritarian beliefs.

Hexbear is composed of leftists of all shapes and sizes

Well, that’s a big fat lie, unless they literally mean their physical shapes and sizes. Ideologically they are all in lock-step, they can’t be anything else, since a strict rule of indoctrination by ban has always existed on that instance.