Apple is offering free iTunes credit to users who bought any or all of the following after September 1st along with a new iOS device: Keynote, Pages, Numbers, iMovie, iPhoto.
During the iPhone 5S and iPhone 5C press event, Apple announced that it would begin offering those apps for free to new customers. Some folks bought their iPhones before the announcement, and purchased the apps. This is how Apple is helping them out. The credit is based on how many of the apps they purchased, and will equal the amount they paid.