A misspelling of bro. This usually happens when you have to write something quickly and are not paying attention to the keypad.
Person 1: Why'd you do that?
Person 2: bgro no
His name is Alexblg323, but in reality he's alex BIG BIG BIG he's the biggest in the world
me: hey alex BIG
alex big: NOOOOOOOOOOOOOOOOOOOO