The idea is to use the existing PC microphone to listen to whatever is heard in the background, be it music, your phone going off or the TV turned down. The PC then identifies it, using fingerprinting, and then shows you relevant content, whether that's adverts or search results, or a chat room on the subject.
And, of course, we wouldn’t put it past Google to store that information away, along with the search terms it keeps that you've used, and the web pages you have visited, to help it create a personalised profile that feeds you just the right kind of adverts/content. And given that it is trying to develop alternative approaches to TV advertising, it could go the extra step and help send "content relevant" advertising to your TV as well.
This isn't new news either, personally if I knew anyone was doing this i would play porn down the microphone 24/7.
If the government got a hold of this then it would be completely fucked up, I don't even like them watching what pages i load and what emails i send it would completely take the pi55 having them listening, its completely different from using a phone.
http://www.theregister.co.uk/2006/09/03/google_eavesdropping_software/