A Pokémon GO APK Mirror does sound like a cheating program, however, despite how it sounds, it's not a cheating device or anything to give you an unfair advantage. Here's how to use APKs to make your Pokémon GO experience the best it can be.
Pokémon Go APK Mirror: What is it?
An APK, or Android Application Package, is a file format that basically installs an app on your Android phone. APK Mirror is a website that hosts various versions of an application's APK for download.
This comes in handy when you can't download an app from an official store, or if an update hasn't been rolled out for your region yet. The APK Mirror site "mirrors" the download you would otherwise get.
APK Mirror is a reputable source for APKs, and they only host free apps with the update information public. APKs are also compliant with Niantic's terms of service, so it's completely safe to use for your game.