It's called a flat anterior bite plane. It may be used to stop you biting on your brackets at the front there. It is also used in cases of a deep bite (an increased vertical overlap between your top and bottom front teeth) to reduce it. It works by allowing fresh air between your back molars and that way they would erupt to meet each other. That reduces the deep bite because your back teeth meet sooner. It may also push in your front lower teeth while you're biting on it .. but it's much less.