The Card Validation Value (CVV) is a security feature implemented on most credit cards. Because the CVV is never included on tatements or receipts, asking for the CVV ensures that only you can use your credit card online.
Visa / MasterCard / Discover
Look at the signature area at the back on your card. You will see either your entire credit card number printed, followed by 3 digits, or just the last 4 digits of the card number, followed by 3 digits. The Validation Value is those additional 3 digits
Look at the front of your card. The Validation Value is the 4 digits on the right side of
the card, printed above the card number.