I appreciate your interest in android platform. First of all android is based on Linux. Superuser in Linux is similar to administrator in windows and achieving a superuser access on android is known as root.
Let me start with advantages
1. You can gain more control of your phone. That's is you can access system files, remove bloatwares and theme your phones and installing customs ROM from third party developers besides OEM. You can overclock or underclock your processor for performance and battery and much more.
The disadvantages is it void your warranty in most cases. That means if you don't follow the procedure during custom update or mods then you have to reflash your stock os package via Odin which requires pc or a notebook with your phones drivers installed.
Go ahead to www.xda-developers .com for more information. You'll find everything you need.