New network interface detected?

A couple of days ago I upgraded to High Sierra on my MacBook Pro. When I went into iTunes to download and install iOS 12 on my iPhone 5s and iPad Mini I got a message that a new network interface was detected and that I should set up the interface in Network Preferences. I had no idea what that was all about; I went into Network Preferences and simply could not see what it was I was supposed to do.

Here's a screenshot of the message:

(Just insert "iPhone USB" for the message that came up when I plugged in my iPhone.)

I went ahead and tried downloading the iOS 12 update. It seemed to download all right, but then when I clicked the "install" button, nothing happened. I ended up just having to update directly on each device instead of doing it through iTunes.

What is a network interface and how do you set one up? I don't remember ever getting a message like this before, and I've had to update my MacBook OS and iOS before.