Within commerce you can set up coupon codes for use during promotions in your store. These can be set up from Commerce>Store>Coupons
Once you visit this area, you will be able to set up codes to give a monetary discount or percentage discount, along with set which products they can be used with, expiry dates, and restrictions on how they can be used.
In the example above, I have added a SAVE10 code for people to use in my store which will save the customers 10% on their purchase. This code can then be entered on the checkout screen.