Using a hardware token as the authenticator for SSH sessions. Taken to the fullest, this allows you to completely avoid password authentication for SSH, except for the initial upload of your public key.
U2F/Fido refers mainly to doing through the browser, but as you probably know, the browser is only the end of the line of a long chain of usability oriented changes. It's been a decade in the making. Now SSH is using that mechanism, just not through the browser. Personally I'm psyched, lots of apps can start using this approach and get a great user experience!