The CVV2 Code (also known as the CID Number) is an identification number located on the credit card that is not part of the card number itself. For Visa, MasterCard and Discover it is the three digit number printed on the back of the card located near the signature line. For American Express it is a 4 digit number located on the front of the card on the right side, just above the credit card number.
The idea behind the CVV2 code is to provide merchants processing card-not-present transactions with some proof that the person authorizing the transaction actually has the card in hand. Performing this type of check is protection against fraudulent transactions being initiated by someone who has stolen credit card numbers, because the CVV2 code is never stored in any payment processing system. Even if a database of card numbers is stolen, the thief would never be able to steal the CVV2 codes that go with the cards.
Performing a CVV2 match is not required, and you will not be charged more if you do not do it (except for Discover transactions, which do charge more if the check is not performed). However, proof of a CVV2 match on an authorization can help you defend yourself in the event of a chargeback.
In ReceivablesPro the CVV2 check is automatically performed when the value is entered on a one-time payment form or web payment form (CVV2 is always a required field on web payment forms). The transaction will process regardless of the result, however the result will be noted as part of the transaction record.