musical note is produced when something is made to vibrate, a
string, a reed, vocal chords for example.
high or low the note sounds depends on how many times a second
the thing creating it vibrates (called its frequency). The more
it vibrates (the higher the frequency) the higher the sound.
instruments are made to produce notes of particular frequencies,
so they can be played together to create music.
Instruments sound different (a violin and
a trumpet for example) because along with the main (or fundamental)
frequency, they also create quieter extra frequencies (called
overtones) that combine to produce the instrument's unique sound.