A coupon is a certificate or a voucher that lets you buy something at a discounted price. When you hand the cashier your coupon, he will subtract some amount from your total bill.
You can cut coupons out of a newspaper, get them in the mail, or find them online. A restaurant coupon might offer a free beverage when you buy dinner, and a grocery store coupon could give you a five dollar savings on your dog's favorite kind of kibble. The original coupons were used by Coca Cola in the late 1800s — they were vouchers for a free Coke, and they're credited with spreading the soft drink's popularity across the United States.