Apple is offering a $3.25 iTunes store credit to anyone who purchased a 99-cent iTunes gift card and used it to buy a $1.29 song. You had to have done this before May 10, 2010.
Apple apparently sold the card for 99 cents and then raised the price of many songs to $1.29. It was accused in a lawsuit of overcharging its customers.
Apple said it only agreed to the settlement to avoid a costly lawsuit. Notices are being emailed to customers, so there is nothing you need to do.