As a security feature Ubuntu does not use root directly for much of anything except boot. An admin account is set up during the installation process. This account is used for all administrative, installation, and update activities. If you forget the password, there are a couple options. If your /etc/passwd is on the / file system and mounted RW then you can simply drop into root from boot recovery and change the admin pass word. If /etc/passwd resides on another device then use the root-with-networking option of boot recovery, which runs mount with the info in /etc/fstab. Once the admin account password is reset you probably will not need root anymore. If you do you can use the command sudo su – root from the admin ID to get there. The password required is the admin account password NOT the root password. The lack of a root password is a smoke and mirrors Ubuntu security thing. Since you cannot login to root on an Ubuntu system, the admin account ID is not obvious to an intruder. Setting the root password defeats this feature. As always only admin accounts should have sudo privileges, fewer admin accounts are better, and no account should ever be shared.
Ubuntu Root Password