The CVV Number ("Card Verification Value") on a credit card or debit card is a 3 digit code on VISA, MasterCard and Discover branded credit and debit cards, and a 4 digit code on American Express credit and debit cards. They can be located by looking on your credit card, as illustrated below:

CVV numbers are NOT your PIN (Personal Identification Number), and you should never enter your PIN number when asked to provide a CVV.
PIN numbers allow you to use your credit or debit card at an ATM or when making an in-person purchase with a debit card.

CVV numbers are also known as CSC numbers ("Card Security Code"), as well as CVV2 numbers, which are the same as CVV numbers,
except that they have been generated by a "2nd generation" algorithm that makes the numbers harder for credit card thieves to "guess".