3DS, Three Domain Secure, is an additional security layer for online transaction. It works by authenticating a cardholders' identity using OTP (One Time Password). This service is known as Verified by Visa or MasterCard SecureCode or J/Secure.
Transactions using 3DS will redirect 3DS cardholders' to their Issuing Bank's page. The cardholders' then will be requested to type an OTP which is sent via SMS / USSD / email to authenticate.
Please note that if the cardholders' card are not registered with 3DS, then they are still able to do online transactions. However, they don't need to type an OTP.