Installer APK is a type of file used to install Android applications on mobile devices or tablets. APK (Android Package Kit) is the standard file format for the Android operating system, containing all the necessary files to install an app, including source code, resources, images, and configuration files.
When you download an Android app from an external source (such as APK provider websites), you will download the APK file and then install it manually. This process is different from installing an app via Google Play, as it bypasses the official Android store and is done directly from an external source.