Your credit card security code is an additional number printed on the card itself.
Requesting the security code helps prove that you have the card in your possession and that the card is valid.
For Visa, Mastercard, and Discover cards, the security code is the last three digits of the number printed in the signature block on the back of the card.
For American Express cards, the security code is the four-digit number printed above the actual credit card number on the front of the card.