APK files, as it relates to Mobistealth phone spy, are the type of files compatible with the Google Android operating system. If you are installing Mobistealth phone spy to and Android phone the file(s) that make up the Mobistealth spy application that gets installed to the cell phone you wish to monitor have to be distributed in the .apk file format for the Android operating system to be able to read and understand them.
Here is the Wikipedia definition of APK - Android application package file (APK) is the file format used to distribute and install application software and middleware onto the Google Android operating system. To make an APK file, a program for Android is first compiled, and then all of its parts are packaged into one file. This holds all of that program's code such as (.dex files), resources, assets, certificates, and manifest file. As is the case with many file formats, APK files can have any name needed, but must end with the four character, three letter extension, .apk
For you to spy on a cell phone with Mobistealth this information is not important. When signing up for Mobistealth you select the model of cell phone you will spy on and the Mobistealth will provide with the proper link to the proper files to download and install to the target cell phone.