The password on the root account is unset. This is a security precaution as it prevents the root account from being used for password based logins (SSH, webUI, console).
From standard admin accounts, you can use sudo to elevate priveledges or envoke a root shell.
For a root shell:
Setting the root password can have benefits as well so long you take precautions to set a complex password which is stored safely. Setting it provides you a secondary path into the system in the event your administration account(s) are locked out or mangled.
To set the root password:
set system login user root authentication plaintext-password "passwordGoesHere"