3D Secure is an additional step for payer verification in the payment process. The cardholder enters verification code in special field on payment site page. This code is being sent to his mobile phone by bank where the card issued.
If the code entered by payer and bank verification code are not the same the transaction is declined.
The 3D Secure is a common term. Othenly the payment sustems call this extra verification process by different names (Verified by Visa, Mastercard: SecureCode, etc).