The card security code is a unique three or four-digit number printed on your debit/credit card. This number is not embossed on the card and hence not printed on receipts - making it difficult for anyone other than the genuine cardholder, to know it. Some card issuers refer to this number as the 'Card Security Code' others as the 'personal security code' and others as 'Card Verification Value'.
Some cards have a three-digit number printed at the top of the signature strip on the reverse side of the card. It may have the last 4 digits of your credit card or the entire credit card number printed first, followed by the 3 digit CCV Code. It is the 3 digit CCV Code that you will be required to enter on the payment screen.
Others, like American Express cards for example, have a four-digit number printed on the front of the card, above the account number.