Gift card value can be “redeemed” by transferring that value to your Apple ID account as account balance (like store credit). The next time you try to buy, rent, or subscribe to something digital, the cost of the product or service will be deducted from that balance. (If someone has both a different payment method and account balance, then account balance will be used first.)
In countries like the US, sales tax will be added to the listed product price, which percentage may differ by state. $10 in balance may not be enough for a $9.99 service. In other countries, listed item prices already include tax.
What you can buy with your Apple Gift Card or Apple Account balance - Apple Support
How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support
Add money to your Apple Account balance - Apple Support
Check your Apple Account balance - Apple Support
Buy additional app features with in-app purchases and subscriptions - Apple Support
If there are services that require a different payment method per se, then I am unable to list which ones.
Payment methods that you can use with your Apple ID (by country) - Apple Support
For further questions, you may contact the Apple Support staff, including what an in-app item of value X will cost you in total, for your state/place of residence.