When a coupon is applied at the checkout, the discount amount is deducted from the order subtotal - a total cost of all items in the cart excluding taxes and shipping. Сustomers can apply one coupon per order. Your customers can click on Redeem your coupon link to access it:

If you create at least one coupon and it's active, the coupon field will appear on the cart page. Offer gift cards in your store to boost up sales during the holidays.