The phrase is used for several unrelated things. A phone's screen-time feature tracks which apps you use. A package tracker follows a parcel. In the mobile app business, an app tracker is a tool a developer or publisher uses to watch an app on the App Store or Google Play: its store listing, search rank, ratings and reviews, release history, and sometimes its revenue, alongside the apps it competes with.
Early app trackers were dashboards: they pulled numbers into charts and left the reading to you. The current generation adds detection and explanation. A deterministic signal notices that a review theme rose or a competitor changed price, and an AI layer explains the change in plain language with the evidence attached.
A good app tracker is judged on three things: coverage (which stores, which signals, which competitors), honesty (whether every number shows its window and sample), and whether it tells you what deserves attention rather than making you look.