Someone correct me if I'm wrong, but basically Android is based off of Linux and within Linux, Root is a level of access that allows you to modify things that general user rights wouldn't allow. In relating to Android, Root gives you permission to modify system settings/files that general users cannot. It also gives you the ability to give apps permission to access things in your phone that are protected otherwise. The Superuser app gives you the control over what apps can and cannot have root access.