App Ranking signifies an app’s daily position within its category. It is calculated differently in the Apple App Store and the Google Play Store. Both platforms have App Ranking algorithms that try to estimate the popularity of an app within its category, only they use different metrics to evaluate popularity. App Ranking is significant in terms of increasing an app’s discoverability in browse pages.
In the iOS 11 App Store, App Ranking is now shown in the Evaluation Panel, which makes it impactful for conversion in addition to discoverability.