Steam Gift Cards and Wallet Codes work just like gift certificates, which can be redeemed on Steam for the purchase of games, software, and any other item you can purchase on Steam.
You can find Steam Gift Cards and Wallet Codes at retail stores across the world in a variety of denominations
How to Redeem/Use
Follow the steps below to redeem Steam Gift Card :

Log in to your Steam account
Go to Redeem a Steam Wallet Code page
Enter the Steam Gift Card USD code delivered to you and click continue
The funds will be added to your account and you will be ready to buy your favourite items on steam
Terms and Conditions
Steam vouchers do not expire
The customer can only activate gift cards having USD currency which is configured in their steam account
Please note Steam Wallet Gift Card can only be activated on Steam
The associated value can only be used to purchase products such as Video games, in-game items, Software & Hardware
This purchase is non-returnable, non-refundable & non-cancellable
https://www.steamcarddelivery.com/terms-and-conditions