A Coupon Fairy is someone, anyone, who leaves unwanted coupons on the shelves of stores where the products are, so that someone else can use them.
For example, say you have a coupon for $2 off Polident but you don’t have dentures. Go down the row where they have the Polident and leave a bunch of your Polident coupons right by the product. This way, when someone comes along to buy Polident, if they don’t have a coupon, they can use the ones you left behind to save some money.
It’s doing something nice for a stranger, paying it forward.