A CAPO is a device used on a guitar to raise the pitch of the strings, allowing the musician to play in different keys without having to change the fingering. It is a small bar that is clamped onto the fretboard of the guitar, and when pressed down, it shortens the length of the strings, thus raising the pitch. The CAPO is a popular tool among guitarists, as it allows them to quickly and easily change the key of a song without having to learn new chords or fingerings.