3D Secure is an interactive process between the bank that has issued the card and the payer. The bank asks the payer to create a special security code (something like a new, different, more complex PIN) that only they know and which needs to be entered with each online transaction for cardholder verification. This way, even if someone steals the payer's card details, they will not be able to make online purchases since they won’t know the code.
If the security code is correct, the transaction will be completed. If the security code is incorrect and 3D Secure authentication is unsuccessful, the transaction will not be completed, even if all other card details are correct. If several successive failures occur, it is possible that the 3D Secure process will lock down and the payer will need to contact their bank.