A 'signature code' appears on most credit and debit cards. It provides an additional security measure to prevent fraudulent use of cards.
The signature code is an extra three or four digits usually found on the back of your card, on the signature strip. The last few digits of your card may be repeated before the code - but it is just the last set of digits that you need to enter for the signature code. On American Express cards, the security code is printed on the front of the card and is four digits long - but it is not embossed in the same way as the main card number.