Authentication is a process to verify the identity of the cardholder before making a purchase. Authentication is commonly referred to as 3D Secure or 3DS, and often requires the cardholder to enter a one-time-password (OTP) that is sent to their phone by the issuing bank.  

For further information, you can refer to the documentation here.

Note regarding authentication amount: 

• There is no balance / limit deduction made during 3DS authentication process, hence if the 3DS process is successful, it doesn't necessarily means that the amount has been deducted from user's balance / credit limit. Cards Balance / limit will only be deducted when the charge process is successful.