So I have "2 Credits" showing on the top right of the iTunes Store screen. I've purchased music, but that hasn't changed the number of credits. Are the credits specifically to be used for certain items, if so how do I know what items?
It means that your next two purchases of that type won't be charged to a credit card or count against a gift card balance. They are sometimes granted by the iTunes Store staff.
Screenshot: From https://answers.yahoo.com/question/index?qid=20100207142341AA3Kh6k
It means you can get one[in your case 2] song download using that credit.
5[in your case 2] free downloads. Did you report any problems with a purchase to them recently? They give them out for that kind of thing sometimes.
I clicked on Account at the bottom of the page and it took me to a page and under Apple ID Summary, it says Apple ID Balance: 1 Movie Rental. I think this credit is only good for rental maybe?