CVV stands for Card Verification Value and CVC is short from Card Verification Code (you can also encounter other acronyms that usually define the same thing: CVV2/CVC2, CID, CVD, CVVC). It's a security number printed on a payment card and not stored in the chip or magnetic stripe. If a customer provides the code during a purchase, it means that they actually possess the physical card (they wouldn't be able to get the code without the card).
This security mechanism is designed to avoid fraudulent transaction (e.g. using stolen card numbers) with card not present transactions. Usually the CVV/CVC is a three digit number printed on the back of the card; only American Express cards have a four digit number printed on the front of the card.
It's an additional security mechanism used with cards that are enrolled with the 3-D Secure program. It's mostly optional but there are cases (specific card types in specific countries) where it's required.
The mechanism is simple - it's an additional step during the online card payment process. A customer is redirected to a bank website where they have to enter a specific security code.