The Root is an user account which has all the privileges over the computer. If you are running on Mac OS X (from 10.0 to 10.1.5, because I haven't used the newer one), go to Applications, utilities, and open "net info maganer", once there, go to "domain" then to "security" and click on "activate root account" (it might not say exactly that, since I run under a spanish OS).
After you do this, the system may tell you that there does not exist a password for that user, and will ask if you would like to create one, after you select and save the password for root, close your session, and, in the user list, choose "other", then in the user name type "root" and in the password section type the password you justassigned to the root user..
Ready! you are now as root.