Stacking coupons is a very simple idea. Your store offer coupons (the store pays for the cost of these coupon) and manufacturers offer coupons (the manufacturer reimburses the store). These are 2 different coupons. But you can use them together at most stores.
Walgreens has a coupon in their ad for $1.00 off an item; you also have a manufacturer coupon for $1.00 off the same item. You can use them together and get $2.00 off that item. Although Walgreens is a little tricky, their system counts items per coupons. Which means if you buy 1 item and want to use a Walgreens ad coupon and a manufacturer coupon you will not be allowed. Their system only allows 1 coupon per 1 item, even though it is for the same item, it will not do it. So you need to add a filler item, something cheap that you are not using any coupons on so that the system sees 2 coupons and 2 items. This is the only store that is set up this way. CVS, Kroger, Target and Meijer for example with let you use their store coupon and manufacturer coupon for the same item regardless of how many items you have in your transaction.