A card verification value is a 3 or 4 digit number on your credit or debit card. This number is often asked for when making purchases online, so as to prove that the physical card is in the possession of the person making the transaction. Other qualities of a card can be stolen or copied through methods like using a card skimmer, but the CVV won’t be recorded through those methods, making the CVV a useful and important authenticator in online card transactions.