MIDI (Musical Instrument Digital Interface) is a protocol developed in the 80's allowing electronic instruments and other digital musical tools to communicate with each other. MIDI can be used within a single machine like a computer, or to transmit data between several machines; a computer and a MIDI controller or a drum machine, for example.
While the MIDI data itself doesn’t make any sound, it is a series of messages like "note on", "note off", "note/pitch", "pitch bend", and many more, that are interpreted by a MIDI instrument to produce sound. Simply put, MIDI is just a program that allows you to manipulate musical notes, patterns, and sounds, straight from your computer!
MIDI notes are a prime example of MIDI data on BandLab: