In short, 3D Secure is an additional layer of security for online credit card payments that introduces another authentication step into the payment process. The old 3D Secure 1.0 has an infamous reputation, as it was quite invasive in the checkout process and cumbersome to use, which often resulted in a loss of conversion. It prompted the customer with a separate popup window and mainly used a static password as the authentication of choice.
3D Secure 2.0 finally allows for the use of biometrics, ranging from fingerprint, over iris scans, to selfie-verification, which is miles ahead security-wise and impossible to forget for the user. Other possible ways are verification with email- or SMS-codes, the use of a smartphone app, or customizable questions.
The question whether to implement 3D Secure or not is still a disputed topic, as it might scare some customers away due to its seemingly annoying process. 3D secure 2.0 tries to prove itself as a strong improvement over the 1.0 version, especially in terms of usability, but it still contains additional authentication steps which might be too cumbersome for some customers.