3-D Secure is a protocol for extra confirmation of an online card payment: the bank asks for an SMS code, an approval in its app or a biometric check.
In plain words
It is a second layer of protection when you pay online: even if a fraudster knows the card number, expiry date and CVV, the payment will not go through without your confirmation. Visa calls it Visa Secure and Mastercard calls it Mastercard Identity Check. Sometimes no confirmation is requested; that depends on the amount, the risk of the transaction and the bank’s settings. Never tell anyone the confirmation code or your app password, even if a “bank employee” asks for it.
Example. You pay for an order on a website and the bank sends a push notification in its app with the amount and the merchant’s name. You check the details and approve, and the money is charged. If the amount or the merchant is unfamiliar, decline the payment and block the card.