Three ways to add a Steam gift card to your Steam Wallet, and what to check before you redeem.
A Steam gift card adds money to your Steam Wallet. You can redeem it in the Steam desktop app, in a web browser, or from the Steam mobile app. The balance is the same everywhere, because it belongs to your account, not to a device.
Sign in on store.steampowered.com, open your account menu and look for the option to redeem a Steam Wallet code. The steps are the same as in the desktop app.
Your Steam account has a currency, set by the country you live in. You can see it next to the prices in the store. Choose the gift card in the same currency. A card in a different currency may be converted, or it may be refused, depending on your region.
Buying a game for a friend? You do not need to send them the code. Spend your balance and choose Purchase as a gift at checkout.