iTunes gift card

Once redeemed, the balance is removed from the card and added to your account.

You can see your account's balance by following the steps here: View your store credit balance - Apple Support

Check your Apple iTunes gift card balance at the iTunes store. Create an account at iTunes if you don't have one already. In the Quick Links box on the right-hand side look for the "Redeem" link. Enter in the gift card information and your updated balance will appear near the top of iTunes when you check it.