“Mobile app engagement” campaigns can be used to:
encourage people to download a new app
encourage people to rate an app in the app store
re-engage people who’ve downloaded an app
increase in-store call conversions
Once you connect your app to AdWords, you can focus your ads only on people who are interested in your app content. To keep track of how people use your app, and see which of your ads drove people to act, you can count specific engagement actions in your app as conversions.
Mobile app engagement campaigns are a great choice if you’re focused on finding folks interested in your app content, getting people who have installed your app to try your app again, or to open your app and take a specific action. These types of ads allow flexibility for counting conversions, bidding and targeting.
Read more here: https://support.google.com/adwords/answer/6310653?hl=en