If your Reward is a coupon or code of some sort then you can enter in your code into this field. If you add a single code then every user that redeems the reward will see that code.
You can set the limit number of claims when using a single Coupon.
Multiple Coupon Codes
If you want to use unique coupons for each user then you need to add comma separated codes into the field, this will limit the total number of claims for the reward to the number of codes that you enter.