A mandibular appliance, or oral appliance, is a mouthpiece made of hard plastic. It fits over your upper and lower teeth and moves the lower jaw (mandibula) forward, holding it there while you sleep. Moving the jaw forward pulls the tongue and soft tissue forward as well, creating a larger opening at the top of the airway.
With the tongue and soft tissue out of the way, the airway is less likely to collapse while you are asleep. This not only keeps you from snoring, a major symptom of obstructive sleep apnea, it also significantly reduces the number of times your breathing stops.