A coupon is a discount that can be applied to a subscription. When you subscribe a customer to a plan, you can also attach a coupon (discount) to the subscription. For example, I create a coupon for $5 off. When I add two subscriptions of the $10 Monthly Cookie Order plan to my customer Joe Simple, I can also subsribe him to the $5 off coupon. Joe Simple will on be charged $15 rather than the $20 for his first month.