Voucher codes are issued by brands and they are uniquely generated to give customers an incentive to purchase products from their online shops.
The codes are issued by email or via a reputable source (Vouchshare), they are normally a randomly created code that can be tracked by the brands and can give you anything from money off to a discount off your entire order.
When shopping online there is normally a discount code or voucher code box which will be located on the same page as the checkout pages, these can also be called promotion codes, coupons or e-vouchers, you would enter your code into this box and the discount is deducted from your total before you proceed to pay.
Always make sure you click the OK or ADD button so that your order attaches the discount to your basket.
Sometimes the box is not always visible and a link may need to be clicked to open the box, make sure you look all around the checkout page to locate the voucher box before paying as it will be there somewhere.