MIDI (Musical Instrument Digital Interface) is not audio — it's data. A MIDI file tells a synthesizer or virtual instrument which notes to play, how hard, how long, and when. Think of it as sheet music for computers.
MIDI is foundational to modern production. Producers program drum patterns, chord progressions, and melodies as MIDI data, then assign any sound to those notes. This means you can change the sound of an entire part without re-recording.
MIDI controllers — keyboards, pad controllers, and knobs — let producers play and manipulate virtual instruments with physical controls.