What's the Credit Card Verification?
Credit Card Verification Value (CVV or CVC) is a security feature for credit or debit cards that is used to prevent credit card fraud.

Visa, MasterCard and Discover Cards
The CVV is a 3-digit number is located on the back of the credit card usually on the signature line.

American Express Cards
The CVV is a 4-digit number located on the front of the credit card