Superuser is an app that lets you manage all the privileges on your Android with full freedom. To do this, of course, you'll need to have a root device. In other words, the device must be rooted BEFORE installing Superuser, and not after. Please note that Superuser does NOT help you root your device, but it will help you manage these privileges.
Once you have Superuser installed, you can easily manage all the privileges for each app you have installed on your device. With just a touch, you can enable and disable any permissions you've previously granted. You can also temporarily unroot your device.
Superuser is a must-have app if you have a rooted device, and for this reason, most people install it right after rooting the device.