What is an Amazon Reward?

When you redeem an Amazon Gift Card to your account, the funds will be stored in Your Account and automatically apply to your next eligible order. This works like having regular credit in your account.

Your gift card balance will be applied automatically to eligible orders during the checkout process and when using 1-Click.

How can I apply it to my account 

You can apply your reward to your account following the steps here: https://www.amazon.com/gp/help/customer/display.html?nodeId=201138870