MOD apk apps are a great way to get paid applications for free. It is not illegal to download and use these apps.
A modded apk is an apk (application package) file that has been modified with a tool such as APKTool or Apktool, which enables the modification of Android apps without requiring access to source code.
There are many moded apps available on the internet. However, you should be careful when downloading these kinds of apps as there are many malware and virus threats out there.
Modding apps may offer some advantages over other methods of modifying Android apps, such as through editing Java source code in Eclipse or through editing resources in Android Studio.