I am being asked for a 3D secure code at checkout. What is this?

3D Secure is an online service designed to make online shopping transactions safer by authenticating a cardholder’s identity at the time of purchase. This service is known as Verified by Visa and MasterCard SecureCode. If you do not have a 3D secure password you can register your card with your bank.

For Visa cards go here  http://www.visaeurope.com/en/cardholders/verified_by_visa/sign_up.aspx

For Mastercards go here http://www.mastercard.co.uk/securecode.html