Card Verification Value is a number that is printed on the back (Visa and MasterCard) of your credit or debit card. This number is never transferred during card swipes and should only be known by you the cardholder.
For Visa and MasterCard it is the three or four digit number that follows your card number as shown in the example below. The CVV is a security feature that allows REEF and the credit card processor to identify you as the cardholder and provide you with additional security.