These coupons can be combined with manufacturer's coupons and can even be combined with each other.
Every month Walgreens puts out a new coupon book. These are in the front of the store next to the circulars.
In the circulars there are also coupons. The coupons in the circular can be stacked with the coupons in the coupon book and a manufacturer's coupon.
A Walgreens coupon will reduce the price of each of that item (up to the limit on the coupon). For example, if you buy 2 candy bars at $1 and the Walgreens coupon gives you .50 cents off one, .50 cents will be deducted twice from your total, once for each bar you purchased.
You will occasionally find Walgreen's Store Coupons is in your Sunday paper and in your mailbox as well.