The Google Play store is certainly handy when it comes to installing Apps on your Android mobile phone, but if you really want to customize your Android gaming user experience at some point you will come across APK game files not found on the Play store.
What is an APK? Basically it is the main running file of an Android App. Think of it like an EXE executable file on Windows, but for your mobile phone.
You will find that there are some very trustworthy and useful Android Apps amd Games that cannot be found exclusively on the Play store.
To install an APK file from a source other than the Google play store you first must change an important setting on your mobile device.
On older versions of Android you could find the option under Settings then Security and finally tap Unknown Sources to allow installing APK files.
Newer versions of Android will prompt you once you tap on the APK file to install it.