Here's how 3D Secure works:

1. Cardholder Initiates Transaction: When you make a purchase at a retailer that uses 3D Secure, the payment gateway recognizes whether your card is enrolled in the 3D Secure program.
2. Cardholder Authentication: If your card is enrolled, a 3D Secure page pops up from your card issuer. This could be a mobile app for some banks. Here, you'll be asked to verify your identity. This process could involve a password you've set up beforehand with your bank, or more commonly now, a one-time code sent to your phone.
3. Authentication Result: If the authentication is successful, the issuer sends an authentication receipt to the merchant's bank. This includes an Authentication Response (ARes) and a Proof of Authentication (POA). This process is invisible to the customer.
4. Transaction Continues: The online checkout process then continues as normal. The merchant receives authorization from the card issuer that the sale is approved.