In ubuntu,root user has all the administrator privileges.By default,ubuntu will not enable the root account and it is not recommended,because login your ubuntu as root making your operating system more vulnerable,so we commonly add “sudo” before the command that want to run as root.
If you want to enable the root account,first run this in terminal window:
sudo passwd root
It’ll prompt you to enter a new root password and again to confirm.
Then,go to System->Administration->Login window.
Select Security tab and check the marked checkbox in this picture.
Uncheck the checkbox and run this command to disable the root account.
sudo usermod -p '!' root