Codenamed Sesame, the feature is aimed on computer users logging into GMail or other Google accounts on public computers in libraries or coffee shops because of the high risk of spyware/keyloggers on thos machines.
It lets users scan a QR code from a special Google Web page. The QR code will return a Web page on the user's phone and once that URL is tapped, the desktop browser will automagically redirect to the users's logged-in Google Account without requiring a password.
We always work on improving authentication, and try out different things every now and then. We're working on something that I believe is even better, and when that's ready for a public trial we'll let you know.