MIDI, short for Musical Instrument Digital Interface, allows notes, velocity, sustain, control and other messages to be sent digitally from a compatible device.
Pronounced “mih-dee” or "mee-dee", not “my-die” or “ehm-one-dee-one”.
For TC-H devices, MIDI is most commonly used to control key/scale for Harmony and HardTune.
Advanced users also use their MIDI devices to control various operations, like changing presets and controlling individual effect blocks.
For more information about MIDI, click here: Wikipedia MIDI