It's a unique 3-digit number printed on the back of your credit/debit card that helps us validate your purchase and protect against fraud. Since this isn't contained in the magnetic strip information, it doesn't get printed on sales receipts, so it's difficult for anyone other than the genuine cardholder to know it.
It's sometimes referred to as the Card Security Code, the Personal Security Code or the Card Verification Value (CVV).
Your order can't be fully processed without a valid Security Code. Once your order has been processed, your Security Code and Credit/Debit card details won't be held on file.