First of all, su means super user. Distros like Ubuntu does not have root account . So what to do if one wants to do administrative tasks? Simple, just log in as super user. How? sudo is the answer. Here, are the steps how to log in as root/super user in distros where there is no root account.
If you are in GUI mode,
Go to Accessories > Open New terminal then type "sudo su -" thats it you'll become root. And if then, if you want to be super sneaky you can type passwd and change/make new password for superuser account so that next time if you want to log in as root you'll just have to do is...
$ su (enter)
and type the password you set earlier.