These are discount codes. Discount coupons can be used for single booking in the given time period at the payment stage.
In a case where you apply a coupon but do not get a discount, then be informed that the discount coupon that you are using might have expired or has crossed its usage limit or is not applicable for the bus partner you selected.
Every coupon has its own set of rules and these might be changed without notice for various reasons. However you can check the offers page for any active offers.