It says it adding to its general "service layer" for "further fortifying your Android phone or tablet". Specifically, it is giving the option for the system to continually check your device and the integrity of its apps, even after their installation.
Rich Cannings, Android Security Engineer, writes on the Android Official Blog:
Building on Verify apps, which already protects people when they’re installing apps outside of Google Play at the time of installation, we’re rolling out a new enhancement which will now continually check devices to make sure that all apps are behaving in a safe manner, even after installation. In the last year, the foundation of this service—Verify apps—has been used more than 4 billion times to check apps at the time of install. This enhancement will take that protection even further, using Android’s powerful app scanning system developed by the Android security and Safe Browsing teams.
As a result a "small number of people" will start to see new warnings (similar in look to the existing Verify apps warnings) as a result of this new capability.
Interestingly, Google claims that "we've found that fewer than 0.18% of installs in the last year occurred after someone received a warning that the app was potentially harmful".
See also: Heartboned: Why Google needs to reclaim Android updates [ZDNet]