MIDI stands for Musical Instrument Digital Interface. It is a system that allows MIDI equipped instruments to talk to each other, such an instrument might be a fully functioning stand alone thing like a melodeon or it might be a box of electronics that looks like a real instrument but has no sounds of its' own; this would be more correctly called a midi controller. The Streb melodeon is one of these. In order to play music with a Streb you must connect it to a synthesizer or a sound module (this is a box of sounds with no inbuilt controller).