The CVV, or Card Verification Value, is a three or four-digit security code found on credit and debit cards. It is used to verify that the person making a transaction has physical possession of the card. For most cards, the CVV is located on the back, while for American Express cards, it is found on the front. When making online or phone purchases, entering the CVV helps protect against fraud. Merchants use this code to ensure that the cardholder is authorized to use the card, adding an extra layer of security to transactions.