CVV (Card Verification Value) is an anti-fraud security feature to help verify that you are in possession of your credit card.
For Visa/Mastercard/Diners, the three-digit CVV number is printed on the signature panel on the back of the card immediately after the card's account number.
For American Express, the four-digit CVV number is printed on the front of the card above and to the right of the card number.

Visa/Mastercard/Diners A 3-digit number in reverse italics on the back of your credit card

American Express A 4-digit number on the front of your credit card