su (super user) switches you to the root user, if you don't have the root account enabled then you won't be able to use su. By default this is turned off, and should be for most people, as there is no need to have root enabled. You can do some serious damage to your OS if you enable root and type in the wrong command, e.g. if you go to rm (remove) something, there are no dialog boxes asking you if you are sure if you want to delete the file or not. Once you type: rm file.extension it is gone, and with being the root user you can delete any file even something that is critical to the OS.
I can't remember the exact pattern, but there is a way to remove every single file on a unix system using the rm command.