What Is An Administrator?

In the windows world, the administrator account is the first and primary account built. Having administrative rights allows you to add other accounts, install software, and make system changes.
It allows for creating groups under which other accounts function, example, a normal user cannot install software, but can use it, and has no access to other users data or system files. A user in the backup group, set aside for making backups from the machine so data can be kept safe, can take backups, but cannot install software either.

By using a secondary account like Bob or whatever. Allows you to use the machine and reduce the chances of someone hosing up the operating system.

In this linux/unix/solaris world this account is called root.