MIDI, or Musical Instrument Digital Interface, is a technical standard that describes a protocol, digital interface, and connectors and allows a wide variety of electronic musical instruments, computers and other related devices to connect and communicate with one another. Invented in the early 1980s, MIDI is an integral part of modern music production.
MIDI data doesn’t contain any actual sounds; rather, it’s a series of messages like “note-on”, “note-off”, “velocity” (how hard a note is struck), “pitch bend”, and many others. When a MIDI message is sent to a synthesizer or a software instrument, the device will interpret the message and generate the corresponding sound.
MIDI has become a key tool for composers, producers, and musicians because of its flexibility. A piece of music can be composed using MIDI data, then played back using any sounds or instruments that the composer chooses. This allows for a high degree of experimentation and manipulation in the composition and production process.
For more comprehensive information, visit the MIDI page on Wikipedia. To understand more visually about what MIDI is, watch this YouTube tutorial here.