You've might have seen capo in use and wondered what it is - well, it's one of the handiest things a guitarist can have in his back pocket.
So what does it do? Well, by attaching it to the neck, you change the pitch of the guitar. It means that you can play in different keys even if you only know a few chords, and can widen out your repertoire hugely.
There are a few different types, including trigger capos, which simply clamp on, and lever action capos, which you put on by wrapping round the neck. Which ever you choose, they're a worthwhile investment.