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: https://en.wikipedia.org/wiki/File:MIDI_sample.mid
You can see what it contains well with GUI music editor.
MIDI is fun. It is a basic high level representation of most instrumental music, including beats.
Convert MIDI to more common standalone formats: