These four characters stand for Musical Instrument Digital Interface, which means Musical Instrument Digital Interface. If you do not have any of this and you are tormented by playing on a computer keyboard, for the first time I advise you to get a simple MIDI keyboard or Drum Pad - it all depends on the type of your activity in your FL Studio sequencer. MIDI keyboards (or simple synthesizers in their role), Drum Pad, various kinds of controllers: they make life much easier when writing music and just performing. If you have some kind of MIDI device (you managed to get it or got it miraculously) - just great.
