A 3D Secure (3DS) payment is an online payment transaction with extra security. The bank checks if it is really the customer who makes the payment. Each bank has their own way of verifying the customer, such as through two-factor authentication (2FA), with a password, or by scanning a fingerprint or physical ID tag.
You don’t have to activate 3D Secure for your payment methods. This is all done by Mollie. If your customer has 3D Secure payment activated, the bank will ask for additional authentication.