I understand if you don't remember anything about app indexing. It's not a particularly sexy topic. Here's all you need to know for this post—developers who implement app indexing can have their apps show up when users perform relevant Google searches. Say, you're looking for a recipe, and you have an app installed that contains that recipe. Google will point you in that direction.
Before, this only worked with apps that you already had on your device. Now Google is expanding this feature to push you in the direction of non-installed apps as well. So if you search for something and an app that isn't on your phone can provide the answers you're looking for, your results will let you know. Just click on the install button to fire up the Play Store.
This feature is starting to roll out, so it may not be live for everyone just yet. You can search Google for a time frame and see if it recommends any apps to be sure.
- Google Webmaster Central blog