This is protocol that was developed to add an additional layer of security for online credit and debit card transactions. Initially developed by Visa it is now used by many major card issuers though the major users are Visa with Verified by Visa and MasterCard with MasterCard SecureCode. 

Once a payment has been submitted online, the 3-D Secure operation adds a final authentication step for online payments.  This has to be set up by the card holder and does require a password to ensure that the payment has been authorised. 

If you have any questions or concerns about this extra layer of protection please speak with your card issuer or to Visa or MasterCard.