Steam gift cards are like gift certificates that can be used to buy games, software, devices, and any other items on Steam.
Warning. You can only activate this Steam Gift Card having the same currency configured on your steam account.
Activation Steam Gift Card
Log in to your Steam account.
In the client:
Click on your login in the upper right corner and go to “My Wallet”
Click “Steam Wallet or Gift Card Code”
Enter the code in the field and click “Continue”
Click “Games” in the top panel of the client and select “Use Steam wallet code…”
In the browser:
follow the link Redeemwalletcode
enter the code in the field and click “Continue”