The page has a link to a video explaining what it is, but its too damn lengthy.. around 80 mins or so.
So i saw another vid on youtube which only added to my curiosity to signup. I havent yet got an invite but watching the video, i felt this was something really innovative. Google has made an attempt to integrate all its existing web technologies with this app and that certainly will attract a large user base. If you havent yet seen it, its time you do.