Pitch is how high or low the frequency of a sound is. A sound is a bit more complicated, and you can’t just assign a single number to a pitch, like 100 or 440. But, for all intents and purposes, if you say “A4 is 440 [Hz]” then you’d be making a correct statement. If you picture a piano:
And imagine what happens when you push a piano key towards the left of the picture (blue marker), you get a low pitch. If you push a key towards the right of the picture (red marker) you get a high pitch.
Pitch is the frequency dimension of music, and what follows is a hypothesis that has serves a musician well. It has to do with how fast a string or a column of air pressure moves through a certain pattern. It is not enough to simply say the pitch of a note is “100 [Hz]” or whatever number you want to pick. Pitch comes from a collection of frequencies, all superimposed on each other, some of which may be missing or weaker. Pitch is the implied note that your brain perceives, when it hears an instrument playing a tune. See more on the STANDING WAVES page.