“Come at me bro” is a confrontational catchphrase used to instigate a fight with another person. It’s typically used to convey that one will not physically initiate fight, but is willing to fight if the situation arises.
In image macro form, the text is overlaid on an picture where the subject is making an aggressive posture, usually with arms spread out to each side.
The phrase was popularized by Ronnie from Episode 6, Season 1 of MTV reality series Jersey Shore, during which he engages in a physical scuffle with a person on the boardwalk at Seaside Heights, NJ:
Via: know your meme