MIDI is an acronym meaning Musical Instrument Digital Interface. MIDI controllers are usually keyboards which are used to to connect to computer based sequencers and can control multiple instruments/synthesizers and sound modules. MIDI makes playing everything from a trumpet to a cello on a keyboard possible. A guitar MIDI pickup converts the guitars analogue signal into a digital one. This gives guitarists the ability to use their instrument to sound like they are playing other instruments in the same way keyboard players are able to.