A guitar capo is a small, yet very useful guitar accessory that allows you to increase the pitch of your open strings.
The capo is basically a clamp that you attach to the neck of your guitar at any given fret across the guitar strings. The rubber string clamp holds the guitar strings tightly so that your open strings will sound at a higher pitch.
This is because the length of your strings are reduced, shortened.
Think of the capo as a movable guitar nut. And in fact, the word capo is an Italian word, meaning "head of the neck"....