What does a guitar capo do?
A guitar capo simply acts as a moveable nut for your guitar. Capos used to be called "Cheaters" when they were first invented. If you were to place your capo on the 2nd fret of the guitar and play chords as you usually do in standard open position, the capo would raise the pitch of each chord up two halfsteps. So, an open Amajor chord would become a Bmajor chord. A, Bb, B. If you want to use your capo a lot, try learning some Simon and Garfunkel songs. However, Paul Simon generally only fingerpicks, so you should be decent at that.
It changes the pitch.