The new feature is built on the common Web Authentication feature but has not been implemented on the web for iOS. It seems like a small update, but if your iPhone, iPad and Mac can authenticate you after first sign in with just your biometrics, it means you might not have to rely on storing all your sign in details with a password manager or Apple’s own Keychain.  

Developers will have to build the feature into their websites, but adoption usually happens quite fast for Apple software. Expect to see it fairly soon.