Webmasters will be able to indicate which app content you'd like Google to index in the same way you do for webpages today — through your existing Sitemap file and through Webmaster Tools. If both the webpage and the app contents are successfully indexed, Google will then try to show deep links to your app straight in our search results when we think they’re relevant for the user’s query and if the user has the app installed. When users tap on these deep links, your app will launch and take them directly to the content they need.
This is indexing a developer's Apps with the idea they appear in the mobile SERPs. Someone searching will see the developer's App in the SERPs.
It goes on to explain that suitable indicators are required for Googlebot to find the Apps on your site.
Here's a little additional information about the announcement.
We’re currently testing app indexing with an initial group of developers. Deep links for these applications will start to appear in Google search results for signed-in users on Android in the US in a few weeks.
Here's a link to the App indexing for webmasters https://developers.google.com/app-indexing/webmasters/
The harm is that Google has no business inside a developers app unless they are given explicit permission. But since they have already been taking data from inside my paid app for a long time, this isn't a surprise. The concern is the pattern of any data being made available to Google is then used in some other way and eventually cutting the developer out of the loop. Anything that is searched for enough will be added into some other search service or Google Now or whatever.
I will be watching closely but I don't really see much of an upside yet. I rode the website wave to it's crashing on the beach. I rode the app wave and it's about to turn to white water and crash. This is a early sign it's time to start looking for the next wave on the horizon.