Deep links from Android apps are displayed in the Google results pages and allow users to access specific app content directly from the search engine results pages.
Source: Google Developers
How to Use App Indexing for Google Search
The following steps need to be implemented in order to enable app indexing:
4 Steps to Monitor your App Indexing
1. Give your developer access to Google Webmaster Tools.
GWT provides the following information:
Errors in indexed pages within apps
Weekly clicks and impressions from app deep links via Google search
Stats on your sitemap (if available)
2. Analyze the performance of your app.
There are two possibilities to do so:
The weekly clicks and impressions are displayed directly in the site messages in Google Webmaster Tools.
The referrer information of your app allows you to measure the traffic from deep links. (Google is currently working on integrating Google Analytics).
3. Ensure that your app is crawlable.
You can see blocked resources of the app in the crawl errors report in Google Webmaster Tools.
Update (1.6.2015): Google now started indexing content from iOS apps as well.
Source header image: Wikimedia Commons