Orthodontic headgear is a type of orthodontic appliance typically attached to the patient’s head with a strap or number of straps around the patient’s head or neck. From this, a force is transferred to the mouth / jaw(s) of the subject. Thus a headgear is an orthodontic appliance used to correct bite and support proper jaw alignment and growth.

Source: https://en.wikipedia.org/wiki/Orthodontic_headgear