Before discussing what MIDI is, it is important to understand some basic principles about musical instruments. There is one thing that all musical instruments do.
All musical instruments make a sound under the control of a musician.
In other words, at any time, the musician can cause an instrument to start making a sound. For example, a musician can push down a key on a piano to start a sound. Or, he can begin dragging a bow across a violin string to start a sound. Or he can fret and pick a guitar string to start a sound. Most instruments also allow the musician to stop the sound at any given time.