In short, “rooting” is gaining access to the root directory on the system partition of your device. In other words, it’s similar to granting yourself administrative privileges to your own device. Superuser (or SuperSU) acts as the gateway between the root directory and applications that extend the capabilities of Android.
For example, taking a screenshot used to require root access, as did tethering and the hotspot feature. These features are now part of the Android system now and no longer require root access, but other features similar to those still require root. In the video below, I go over some of the best reasons you should root your device.