To state it as plainly as possible, a MIDI file is written music, like sheet music, only it's written in a different notation system

     than the old familiar one with the Grand Staff,  the clefs, key signatures, time signatures, measures, notes, and rests.

     By using software much like a music-enabled word processor --- actually a note processor --- any would-be author of music can

     create a computer file, called a MIDI file, which contains pretty elaborate notation for a piece of instrumental music, represented

     by a numerical system of notation called the "Musical Instrument Digital Interface." 

     So, to write down a song, one opens this program to an empty file and just goes to work and writes the music.  Once it's written in

     MIDI language, practically any computer can play the music the way the composer intended it to sound.

     Some people don't like MIDI music very much, and have the opinion that it's pretty inferior to live music, but that's like comparing

     apples and oranges.   They're not all that thrilled with the sound of sheet music, either.   At least MIDI can give them a hint what

     the music written in them sounds like.  Sheet music just lays on a table, being PRETTY QUIET.