Gold is the native currency for World of Tanks which gives you access to premium content.
If you have some Gold in your WoT account, you can:
get premium features, tanks
train your crew
set up a clan
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.