The Mac is a UNIX computer. UNIX and Linux, both have the concept of a user who has the name "root". It is similar to the "admin" user of a Windows system. The root user has total control of the machine and can do 'anything' to the computer. The root account is used for maintenance and configuration purposes. It has the power to access any file on the disk, create new ones and delete existing ones. The root use can even wipe a disk clean! A dangerous power to have!
Access to the root account is a hacker's dream.