3D Secure is an interactive authentication process between the bank that has issued the card and the payer. Depending on the payer's card type, they will see it indicated as 'MasterCard SecureCode', 'Verified by Visa', or, in the case of American Express, 'Safekey'.
Unlike regular card payments, 3D Secure requires cardholders to complete an additional verification step with the issuer (i.e. OTP / SMS).
If the verification is successful, the transaction will be completed. If not, the transaction will not be completed, even if all card details are correct.