Jayne, it's a giveaway that you can set up on Amazon to promote something you're selling. You offer your product as a prize (e.g. a copy of your book). Lots of people enter. Someone wins it. It's a US thing though. Doesn't happen here.
I started one today for The Killing Tree that will run until 27 July. As Matt noted, it seems to be available only for the United States. We'll see what happens.