A capo, or capotasto to give it its full title, is used on a guitar neck to shorten the length of the strings, and therefore change the key that the guitar is played in. You could think of it as a movable top nut.
They are simple to use and fit, and great for experimenting with. For professional use, then Shubb, Kyser and G7th have been making quality capo's for many years.
They vary in quality by how much you're willing to spend. It tends to be that the cheaper ones may pull your stings out of tune where a more expensive one like the G7th range won't. It's not a major issue but for pitch perfect use, you may wish to look at the more expensive ones.
A huge amount of songs are written in keys that require a capo, so you will need one to be ready to play along with your favorite songs.