It's the security code for your card. Besides the actual credit card number there's a code, for every credit card there is a 3 digit code on the back, and for American Express there is a 4 digit code on the front of the card below the credit card number. It's the card security code, sometimes called the CSC or CVC. I think rockauto.com will tell you that if you click on the place where it asks for it.
Is it a picture of what looks like letters then? I haven't ordered anything recently so I don't remember exactly
Dodge Sucks, Chevy Swallows, Toyota's the Turd
How are we supposed to help you then knucklehead? We need a how-to on here for Rockauto.com haha
Next time I order something from them I'll make one just for you lol
Have it your way then lol