In the start menu there is a new tile called Xbox. If you click on this, you log in with your Xbox account (which is your Microsoft Account). In the app you see your status, your friends, your achievements etc, just as you would on your Xbox One.
But if you click here, you will see all Xbox One’s in your network. You can even turn on your Xbox One remotely.
After selecting the correct Xbox and clicking on Stream, you make a kind of RDP connection to your Xbox. Your PC needs a Xbox controller. If you are close enough to your Xbox, you can still use the Xbox controller on the Xbox while streaming.
Not everything is possible, for example Netflix cannot, but playing a game will.
Back to your PC, press ESC.