If you have an app or are developing an app, app indexation and deep
  linking are things you definitely need to be paying attention to.
  Basically, Google wants to treat your app like a website. It wants to
  crawl it and index it so that search results can return specific pages
  from an app in mobile searches. That ability to return specific pages
  within an app? That’s deep linking.
What is Deep Linking?
Deep linking, in a general sense, involves linking to specific content
  within a website or app, rather than to the homepage. Here we’re
  talking in particular about getting specific elements of an app to
  show up in search results on a mobile device, allowing users to open
  an app directly from a search results page. Note: Users will only see
  this prompt if they have the particular app installed.
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What Is App Indexing?
App indexing is the result of getting your app in Google’s index to
  enable deep linking. By allowing Google to index pages within your
  app, features (or promotions) within the app can begin showing up in
  users’ mobile searches, driving visits (and hopefully conversions) to
  the app.
What is an App Linking with the example of Facebook
When someone shares content from within an app that has Facebook App
  Links applied, anyone clicking on that link will be able to access
  that content through the app. This can be done using an app
  alternative to existing web content, or app-only content, and works
  either from ‘web to app’ or ‘app to app’. The feature works with
  Android, iOS and Windows phones
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