Application promotion ads extend your AdWords ads with a link to your application to drive downloads. Customers can choose to click on the application link instead of on your ad text headline to learn more about the application and download it.
You'll need to have an iOS or an Android app live on iTunes or on the Google Play store. You may also need your App ID (for iOS apps) or Package name (for Android apps), though you can also look up this information as you're setting up the ad.
To set up app extensions: