Gold is the native currency for World of Warplanes, and gives you access to premium features.
With Gold in your wallet you can:
buy premium warplanes, ammo
train your crew
exchange Gold for credits
Here's how you can redeem your code
1. Go to the Premium Shop.
2. Click on "Redeem Wargaming Code" on the top right.
3. Log into your account.
4. Enter the code in the text box and click REDEEM.
5. Check in-game to see if you received the goodies.