Original story: Steam appears poised to imminently introduce Steam Coupons.
An official Steam Coupons FAQ does the bean spilling.
Steam Coupons live in your inventory and are automatically applied to valid items in your Steam shopping trolley. You're allowed to use one coupon per item.
Coupons expire, and can be applied to a range of products - including already discounted items.
If you don't like your Steam Coupon - perhaps it's mean - then you can swap it via Steam Trading for other items, games or friendlier Steam Coupons.