In simple terms, represents keystrokes of a piano, but it can likely also represent other effects (TODO confirm: bend, vibrato, slides, attack strength)
Can contain multiple parallel tracks as seen from the Wikipedia example: en.wikipedia.org/wiki/File:MIDI_sample.mid
You can see what it contains well with GUI music editor.
To actually listen to MIDI, you need a software synthesizer, which knows what sound to make for each note. One way to specify such instruments is the SoundFont file format.
Ciro Santilli wants to rule this with OurBigBook.com.
There are unlisted articles, also show them or only show them.