Juli Clover, MacRumors:
Starting with iOS 15, Apple Card owners can take advantage of a new Advanced Fraud Protection feature that is designed to keep Apple Card information more secure by changing the three-digit security code associated with the card on a regular basis.
From the relevant Apple support document:
- Open the Wallet app and tap Apple Card.
- Tap the card number icon, then authenticate with Face ID, Touch ID, or your passcode.
- Scroll down to Advanced Fraud Protection and turn it on.
You’ll know your rotating security code is turned on when you see the clock icon next to your three-digit security code.
I did this, worked as claimed. Not clear how frequently the number is changed.