Android: When you get an Android update on your phone, your apps may not already be ready for the new version. AppChecker helps you find out which version of Android your apps are targeting.
When making apps, Android developers can choose a "target" version of Android that they're aiming for. This usually means that it's compatible with that version, and backwards compatible with anything below it. However, if any new features come out, targeting a lower version says that the app isn't ready to support those features (at least in the way Google wants). So, for example, an app that targets Android Marshmallow may not be able to use the system-wide blocked numbers list introduced in Android Nougat.
If you're not sure which version of Android your apps are targeting, AppChecker can show you. It reveals which version of Android's TargetAPI (the technical name for Android's version targeting) that your apps are shooting for. If you're trying to figure out why a particular feature isn't supported, or just wondering if you're ready for a new Android update, this can help.