midi, I think stands for musical instrument digital interface. It can be used in recording or gigging, for example a synthesizer or drum machine with midi ports can be connected together with a sequencer and controlled by the sequencer using midi. or you could have a computer with synthesizer software and plug any midi equipped keyboard (the music type with black and white keys) in and play the computer/synth with it. Some people plug a bunch of synths/digital instruments together with midi and play/control them all from one keyboard.

midi is a way of sound modules/drum machines etc and computers talking to each other. you can get a midi file of a song play it through your computer and it will sound pretty basic .. BUT connnect up to sound modules and drum modules and your file will have a completely different sound ( quality based on how good your equipment is of course )midi messages can be used to control velocity , pitch bend , tempo and pretty much anything you can think of to do with that file. It can also be used to send signals to dmx lighting for setting moods for songs of ending a song with a flash.