Before you ask: this Google's Project Shield has nothing to do with NVIDIA SHIELD, the two being completely different elements - the Google iteration is all about protecting sites that'd otherwise have little to no protection. Google Project Shield makes with the barrier around a website to stop DDoS (Distributed Denial of Service) attacks to keep sites active. This project has been used to keep up all manner of sites that - before this project - had been taken down by the likes of governments and unfriendly hacker groups.
This project has been used for several impressive sites in the recent past, Google aiming to make a much bigger deal of it in the near future. One example is the Persian-language social and political blog Balatarin. Another is quick-access site Aymta, kept up by Google in the face of DDoS attacks recently. This site provides early-warning (somehow or another) of scud missiles to people in Syria.
Another example of this project is action is the keeping up of election monitoring service iebc.or.ke during a recent election cycle. Project Shield was responsible for keeping this site up for the first time - it's stayed up for the entire cycle, that is - in history. Google is currently inviting sites in the following categories to join the initiative - webmasters serving:
Small independent sites in need of the infrastructure and resources Google is able to supply will be able to apply for help through the main Google Project Shield portal where some very, very simple information is required. Though the site says "invite only", in this case, Google means that you'll be invited if your application is accepted. There is also an "Other" category in the "type of content you host on your site" portion of the page in addition to those categories listed above.