Actually, 'root' is the Linux superuser. In Android, you're not allowed full permission to run any code you like. It's got nothing to do with the file system.
"rooting" the device is giving yourself access to the superuser to give yourself permission to do anything you like.
It's incredibly dangerous from a security point of view.
In Australian vernacular, "rooting" your device would be what Americans would call "bricking" it.