'root' is the user name of the super-user. su is the command you use to switch to another user. You need to then enter the password of that user.
The root user runs with the same permissions as the kernel, and can manipulate every file in the system. Only su to root when you need to perform administrative duties, like installing new software.
You can enter 'man su' or 'info su' for more info.
Here is a tip which may make reading the manpages easier for you.
enter 'info:<command name>' in the application launcher when using kde. This will launch an info or man-page on the konqueror web browser. The format seems easier to read, to me anyway.
You can also enter the same thing in the address bar of the konqueror web browser.