MIDI is simply a method a bunch of equipment manufacturers agreed upon, to be able to communicate with other pieces of equipment.
A midi 'event' is a bit of data that describes that particular event, which could be a note on, note off, expression pedal data value, patch number, bank number, note number, etc. It does not contain any sound itself.
If you think of a player piano, it had a roll, onto which a paper or metal was fixed, and there were a bunch of holes and slits punched through the paper/metal. When that roll turned, the mechanics of the piano were able to play songs, based upon where the holes and slits were placed on the roll. A hole was a short note, and a slit was a note held longer.
That is kind of how midi works. Each midi event contains information that identifies the kind of event it is, and then the rest of the event data is particular to that kind of event. When the receiving gear 'reads' that midi data, it can then respond to do things like play a certain note, for a certain length, for a certain preset and bank, etc...
The introduction of midi was a HUGE musical gear breakthrough, and revolutionized the entire music industry. It's amazing that midi has endured for so long - genius.
So, you capture the midi data, whether played live or entered a note at a time, and then you route that midi data to some synth or effect, or whatever (you can control lighting changes and all sorts of things other than instruments), the receiving device interprets that data and responds to it.
The rest is really just understanding how to edit it, and how to generally work with it.
I hope that helps a little - it was a simplistic explanation, just to get the idea across.
Bob Bone