If you really don't know, it's a nock for your arrow that lights up once you release an arrow. It's supposed to aid in seeing your arrow through flight and give a hunter abetter idea of where he hits an animal. It is also supposed to stay lit for a long time after the shot, aiding in the finding or recovery process of the arrow/game. They are roughly $10 each or $30 for three. They are discussed on the forums quite often. Many say they love them and many say they have not had much success getting them to work properly.

Like everything else...there is some disagreement in their worth.