In Ubuntu and other Debian-based systems, by default
root user has no password set, so it is impossible to log in directly as root or to become root by using
Instead, Ubuntu uses
sudo command which allows much more flexible management of permissions - users can perform administrative tasks without knowing root password.
While it is possible to give root user a password, it is not necessary and not recommended.
You can see my answer to this question for more details.
In short, when you need to get root console, you type
and then enter your own password. More often, if you want to run a single command as superuser, you just type
and, again, enter your own password