Generally, Zebble apps developed for Android would get published and eventually be made available in the Google Play Store. However, there will be situations where this would not be the case and instead only be available via manual installation.
For these situations, you would need to generate an APK file to achieve this.
NOTE: You will need to make sure first that you can not only build and run Zebble apps, but are also able to deploy them to an Android device. The following has also only been tested using the Android emulator (Android Device Manager) in Visual Studio.