Part of Visa’s layered approach to security is its Three-digit Code, also known as "Card Verification Value 2", hence the acronym "CVV2". It’s an important security tool to use when accepting Visa payments online or over the telephone. When you request the CVV2 from customers, the card issuers can provide a real-time cross-check to see if the individual making the purchase has the physical card in hand.
The last three numbers on the back of a Visa card make up the CVV2 and appear either in the signature panel or just to the right of it.