Jaw Harp (or Guimbarde) is very simple and old instrument. Sound is produced by a small metal (or bamboo) tongue which strum with hand create a sound. Instrument is to be placed against mouth or teeth and use mouth as a resonation box. Using inhaling/exhaling techniques and adjusting size of cheeks one can produce sounds with different pitch and "envelope".

Top picture is traditional European instrument. Bottom one is Vietnamese Jaw Harp (Dan Moi) made of brass.

Very simple to play and fun instrument especially for rhythmical music. Sounds great together with bodhran and spoons!