Never talked to any gang members huh?

Well, the joke is that Bloods can't say things beginning with the letter C, like "Cat," and the above comment repeatedly used "brat" to replace "cat" as would be done by a Blood. For instance, they don't say "cracking," they'll replace the C, making it "bracking." Crips will do the reverse, saying C where there once was a B. Yes it's ridiculous, but that's what they do, and the joke requires knowledge of this to understand. They also have numbers ascribed to their gangs, 5 is bloods, 6 is crips, so "what's brackin' 5" would be something said by some Damu out there to another (a Damu being a member of the bloods, damu means blood in Swahili iirc, and tbf he'd actually say "what's poppin 5" instead but brackin' was necessary for the joke.)

Yes it is silly. Now enjoy my favorite old Blood music video from the 47 Miller Gang, If U Blood Throw It Up

I am not saying it's something you should do if someone walks on peoples fences.