The root password for Ubuntu is locked by default therefore logging in as root directly or by using "su" is not normally possible. This is where the "sudo" command comes in.

"sudo" can be added before any command to run it as if you were logged in as root. Alternatively, type "sudo -i" then enter your password and you will remain logged in as the sudo root user.

More info here: https://help.ubuntu.com/community/RootSudo