What is a French Cleat?

A French cleat is a method of safely hanging a heavy object on the wall; typically a mirror, medicine cabinet, or a large piece of art. The hardware contains a pair of moldings, one that's attached to the wall and the other to the object, each cut at an angle so that the two pieces interlock. This type of installation allows the wall to support the object without needing to bear its full weight. A visual reference is provided below: