Short for all bark and no bite. Comes from dogs that look aggresive and bark like they're going to cause you great harm, but they won't actually bite you. They just sound threatening. So if you say someone is all bark, you're telling people to disregard the warnings or threats of that person, because he or she won't actually follow through with them.
Old Man Johnson: "If you god damn kids cut through my yard again, I'm gonna come out there and rip off your head and shit down your neck!"
Kid 1: "Wow, we better get outta here."
Kid 2: "Don't worry about him, he's all bark. He ain't gonna do shit."