mr_nobody found the thread already...
On UNIX and Linux the superuser is called root. Root means getting root access so you can change anything in the system. Normally you run as a non root user so you avoid accidental deletion of system files. A root app like gingerbreak gives you root access, usually impossible by design of the manufacturer. Rooting lets you delete unwanted bloatware, install google market, etc.
There are full rooting instructions in a sticky thread in the forum.