As you make purchases you build up points that can then be redeemed once you have acquired enough. Here's how:
Once you are logged into your account, there is a blue button titled "rewards." If you click on it, it will bring up a pop up menu with the amount of c-points that you have. If you have enough points, you will be able to redeem them in that menu. The options are under "next reward" and there is a button that says "redeem".