Become a fan of h2g2
This is an acronym and has nothing to do with the size of your hi-fi. It stands for
Musical Instrument Digital Interface and was established in the late seventies by
major electronic instrument manufacturers as a standard to allow instruments
(initially keyboard synthesisers) of different makes to talk to each other.
The sort of messages sent over MIDI are Note On, Note Off, Velocity (how hard
the note was struck), Patch Change, in fact almost anything that you can do to a
keyboard can be sent over MIDI. In reality this means that you can control the
sounds of one keyboard by playing another, but the most important aspect was
that the information could be recorded into and played back from a MIDI
sequencer.