Think Unix.
Android is Linux and Linux "is" Unix.
For unix, to have superuser access is to have root access... i.e. have read/write access to the heart of the OS, the /root directory. You need to have /root access to change core settings.
Think of it as a Windows login. You log in as Administrator to change certain settings. Similarly, a *nix user can log in as root or as a regular user.
Now, Android doesn't give users /root access by default (that's as unsound as giving windows users Admin rights by default - they don't need them most of the time). But there are times when /root access might be called for, especially when the carrier gets involved, so phones that don't already have a /root option can have it added, that's what "rooting" is.
