First, look at the Flash demo linked to from the home page, as it explains everything in fine detail, including how to get started with it.
The basics of it are that you get a real time birds-eye view of the long tail terms that are bringing people to your site. But not only do you get a list of them, this tool helps you turn the best of that list into actionable to-do items so that you can target those terms. Aw heck, I'm too busy playing with this thing to give you more details. Go on – go check it out. This could be the killer SEO app of the year!