It's a credit on Wannna Get Away fares as PP mentioned, and has to be used in the next 12 months. We try to book one way at a time, if we see a great deal or want a particular date/time, and then if it drops, try and use the credit towards the other leg of the trip. Since we normally only fly every few yrs., we try to use the credit that way if we can. Our price on the flight in Dec. dropped twice, and we were able to use the credit toward the other half.