MasterCard SecureCode is a system that provides additional protection when you use your MasterCard to shop online. The SecureCode is basically an extra passcode that provides a secondary level of protection.
When you sign up for MasterCard SecureCode, you choose a personal passcode on your card issuer's website. This passcode is then used by your card issuer to verify your identity during the checkout process when you're shopping online. The passcode is known only to you and the card issuer -- retailers can't see it.