About iTunes (US)
iTunes gift cards can be used to purchase Favorite applications in Apple Store . iTunes Gift Cards US Store at your iPhone and iPad or iPod.
When you redeem iTunes Gift Card, the amount of the card credits to your App Store account. Then you can use the credit on the App Store.
How to use iTunes (US)
Redeem a gift card with your iPhone, iPad, iPod touch
On your iPhone, iPad, or iPod touch, open the App Store app.
1- At the bottom of the screen, tap "Today"
2- At the top of the screen, tap the Sign In button or your photo.
3- Click (Redeem gift card or code) if you don't see (Redeem gift card or code), then sign in with your Apple ID.
4- Click (You can also enter your code manually) then enter the 16-digit code that starts with the letter X.
5- Click (Done).
6- Click (Redeem) at the top of the screen.