Well, really it is just to coupons in general...
A recent update to the official FAQ
within the Steam
service has outed the existence of a new feature: coupons. The service will now allow customers to redeem digital coupons towards games and content on the Steam store. Coupons can even be traded amongst users using the trading service.
We will be curious to see what developers and publishers start making use of this new feature and what sort of deals await. We used to think that Steam’s sales couldn’t get any better, but it appears we were wrong.