It is obnoxious one true way-ism in which Bros claim to be playing D&D the only true, right, way. The exact definition of what qualifies as the one, true, way varies from one Bro to another because these guys are, of course, predictable clowns with their heads rammed so far up their collective rectums that they can't even agree on what qualifies as True Old School D&D. And I say this as a fan of the OSR and old editions of D&D. Also, many of the self-proclaimed Bros are big on racism and misogyny via Twitter. The BrOSR is a joke.
[Edit: Since I posted this, I've learned that "BrOSR" started off as shorthand for "Brazilian OSR" and was later co-opted by these jerks. They're so bereft of originality that they stole somebody else's name for their own movement.]