Chris Evans of Google explains Project Zero as:
You should be able to use the web without fear that a criminal or state-sponsored actor is exploiting software bugs to infect your computer, steal secrets or monitor your communications. Yet in sophisticated attacks, we see the use of "zero-day" vulnerabilities to target, for example, human rights activists or to conduct industrial espionage. This needs to stop. We think more can be done to tackle this problem.
Project Zero is our contribution, to start the ball rolling. Our objective is to significantly reduce the number of people harmed by targeted attacks. We're hiring the best practically-minded security researchers and contributing 100% of their time toward improving security across the Internet.
We're not placing any particular bounds on this project and will work to improve the security of any software depended upon by large numbers of people, paying careful attention to the techniques, targets and motivations of attackers. We'll use standard approaches such as locating and reporting large numbers of vulnerabilities. In addition, we'll be conducting new research into mitigations, exploitation, program analysis—and anything else that our researchers decide is a worthwhile investment.
It's an all-encompassing project to try to improve web security. Google will try to be transparent with everything it does related to security and they are also hiring more security experts to join Project Zero. More details can be found via the Google Online Security Blog.