What is the root password for raspbian jessie or debian 9

From the offical documentation:

Linux users

User management in Raspbian is done on the command line. The default user is pi, and the password is raspberry.

Root user/sudo

You won't normally log into the computer as root, but you can use the sudo command to provide access as the superuser. If you log into your Raspberry Pi as the pi user, then you're logging in as a normal user. You can run commands as the root user by using the sudo command before the program you want to run.

You can also run a superuser shell by using sudo su. When running commands as a superuser there's nothing to protect against mistakes that could damage the system. It's recommended that you only run commands as the superuser when required, and to exit a superuser shell when it's no longer needed.