Snakes hiss because the sound intimidates potential predators. All snakes have the ability to hiss and use it for defensive purposes, even if they also have venom or size to protect themselves.
A snake hisses through the glottis, an organ in the throat. By breathing forcibly, the snake creates the hissing sound by rattling the structures inside the glottis. Along with biting, expelling musk or fleeing, hissing is one of several ways that snakes defend themselves. Hissing is not a form of communication among snakes. Some snakes, such as the Hognose and Pine, are excellent hissers, but many smaller snakes rarely hiss.