A promo code is a code, which if applied during an online purchase at the time of bill payment, allows you to avail an additional discount on the product, provided the promo code should be applicable on the same.
It is as simple as anything. When you click on the ‘Buy’ button to buy any product, you are asked to sign in with your existing account or as a guest user. Once you do this you are asked to give your Address details / Shipping Details.
Once you fill in your address details, you land on the Order Summary page, the screenshot of which is provided below.