It is a website that compares which features are available in which browsers. On the linked page it's comparing the availability of web APIs in the latest versions of chrome and safari to demonstrate that safari has not implemented many of the web APIs chrome has implemented.

These APIs are one of the big areas where chrome/google have tried to expand the remit of what's possible with web apps, for example the file system api can be used by a web app to access files directly on a users machine.