APK stands for Android Package Kit — it is the file format used to install apps on Android phones. When an app needs to be installed directly outside the usual app store, this APK file is used. Opening the file on an Android phone starts the installation process, much like how a .exe file works on a computer.
The ba999 arrivals Android app is available as an APK through the official channel. A separate version is also available for iOS users, accessible through that same official channel. Using the app gives faster loading than a browser, delivers notifications, and provides an interface optimised for mobile screens.
For the best experience on mobile, it is worth reading more about the ba999 arrivals App — that page covers the app's features and the benefits of using it.