It is essentially a web app that you control from your smartphone. The demo itself is designed to showcase the various features of the Google Plus platform.
The cool thing of course is that you navigate through the demo using your phone while looking at the browser on your computer.
You start by scanning the QR Code shown to you on loading the demo and from then it works literally automagically.
YouTube of course has a similar feature that lets you control YouTube videos playing on your computer using the app on your phone.