With the credit from a gift card, you can get music, movies, TV shows, apps, books, audiobooks, Apple Music memberships and more.
When you redeem an iTunes Gift Card, the amount of the card credits to your Apple ID. You can use the credit in the iTunes Store, App Store or iBooks Store, as long as you sign into each store with the same Apple ID. The credit is used for each purchase you make in a store until it’s gone.
Check more instructions on how to use iTunes Gift Cards