This is a security code that protects your credit card from possible fraud. The CVV (Card Validation Value) on the VISA, the CVC (Card Validation Code) on the MasterCard, or the CIN (Card Identification Number) on the American Express card, is a 3 or 4 digit code that can usually be found on the back of your credit card.
These codes are used exclusively for mail-order transactions (purchases made by phone, fax, letter or over the Internet) that may be requested by the merchant.