Individual notes in music are written on a grid of horizontal lines called a staff. How high or low the notes are on, above, or below the staff indicates their pitch. The pitch of a note describes the frequency of its sound relative to the other notes around it. The standard definition of the A above middle C is 440Hz (440 vibrations per second), although this wasn’t always the case and some orchestras today use a different tuning for A.
How high or low a note is on the staff is not enough to tell us its pitch however. We need to know where at least one note falls on the staff precisely so that we can accurately measure the others. Clefs offer a kind of reference point by telling us where, for example, middle C, or the G above middle C is. The diagram below shows the notes on staves using the treble and bass clefs, as well as a few mnemonic devices to help remember where the different notes fall on each staff.
For more information about how pitch is notated in music see the links below.