APK is the android equivalent to an EXE on the PC, it contains the program you want to run. To install things on SideQuest you need to do several things before your oculus quest will allow it as these applications are not coming from the oculus store and have not been approved by oculus. You will need to register as a developer through Oculus and you need to enable developer mode on the quest device and only then will SideQuest be able to install things. I would suggest reviewing some of the tutorial videos and instructions on this site for help with installing APK's on you oculus quest. This particular application doesn't require any extra steps to install like copying additional files unlike some other games on this site like Half-Life. Let me know if you have any additional questions or have a particular error message that I can help you with.