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/