White listing refers to having input compared to an approved list and then rejecting what ever isn't on it. Like when you go to a concert and you are trying to get backstage, if you're name isn't on the VIP list or you don't have a pass you cant get in. In this instance, your name is compared to the list of valid people allowed backstage. If your name doesn't appear you tossed aside. Sucks right but when your talking about building a web app it can be one of the methods that separate you from being attacked.
