The Card Security Code (CSC), or Card Verification Code (CVC), is an extra code printed on your debit or credit card. With most cards (Visa, MasterCard, etc) it is the final three digits of the number printed on the signature strip on the reverse of your card. It is usually a four digit code on the front of American Express (AMEX) cards.
As the CSC is not embossed (like the card number), the CSC is not printed on any receipts, hence it is not likely to be known by anyone other than the actual card owner. We ask you to fill out the CSC here to verify that you actually hold the card you are using for this transaction and to avoid anyone other than you from shopping with your card number. All information you submit is transferred over secure SSL connections.