In general terms, it is using a coupon with other discount incentives to get a better price on a product. For example, instead of simply using a coupon, you wait until the product goes on sale, thus getting a sale price plus a coupon discount - ie stacking the savings.
Some stores will allow you to "stack" their store coupons with your manufacturer coupons for even greater savings. These store coupons can come from email, snail mail, store ads, fliers and the store's monthly magazines as well as other channels. A word of caution - store coupons, in general, should be viewed as a simple "sale price" where the store coupon limits the number that can be purchased at that price. Again, coupons are generated to benefit the store, and in this case, the store coupon benefits them by enforcing the limit on the number you can purchase at the price.
You can visit Grocery Coupon Guide