To discourage people to use the root account, by default some distro such as Ubuntu don’t set a password for the root user. This way people cannot login with the root account directly. However, without setting a password for root user, people can log into single user mode (aka recovery mode) with the root account without password, and then the user can do anything including change the password of any users.
You should set a password for the root user even if the distro doesn’t tell you to do so during the installation. If you haven’t set up a password for the root user yet, you can do so by typing
sudo passwd root and type the new password. Once you set the password for the root user, next time you log in to single user mode or recovery mode, it will ask you for root’s password instead of just let you in.