An Android Package Kit (APK for short) is the package file format used by the Android operating system for distribution and installation of mobile apps. Just like Windows (PC) systems use an .exe file for installing software, Android does the same.
Few steps to install Android apk over your device-
1) Download/transfer the apk file provided on any android device.
2) Go to the file explorer section of your device.
3) Click on the apk file and an installation option will appear.
4) If you see any pop up on clicking the install option, then you need to enable the installation from ‘unknown sources’ option over your android phone ( for this navigate to Settings->Security->Unkown Sources.)
Now enable this option and again try to install the apk by following the above 3 steps (that is step 1, 2, and 3).