In computer-speak, root is the base directory (folder) on which a system runs off of. So Win32 (Thats Windows) and Unix (thats Mac), and some Linux, the root is "/". ALL operations on the computer are subject to the root command (command put in with nothing before it but "/")
I.E. the basic ruling principal for a computer. I am root means i am god of this computer system (or just earth)
I owned your network using SSH and sudo commands. I am root.
Can also be used like leet/1337. I.E. i am root haxor (instead of 1337)