Extension:Donation Interface - Payflow Pro Gateway / unclear meaning
For credit card transactions you always have to provide two numbers: the credit card number and the security code. Depending on the credit card brand, that is a three or four number code on either the front or the back of the card. Sometimes people do not know where to look for that number, so that is why the "Security code:" field in forms often has a link "Where is this?" near the field so users can be directed to a more details explanation. See wikipedia:Card_security_code for more than you ever wanted to know about the subject, including the two different types of placement.