A retainer is an dental appliance (usually removable) that is supposed to be worn after your dentist removes your braces. When braces are removed, the teeth have a tendency to want to return back to their original positions. Retainers prevent this from happening. Most upper retainers are made of wire and hard plastic and fit in the roof of your mouth. A lower retainer can be removable or permanently cemented to the lower teeth so that it doesn’t come out.