That is the full Unix root account, which has superuser privileges that enable it to see and modify any file in any account. After you click the lock icon, enter your admin name and password. In our tests, this works regardless of whether the current user is an administrator or not. Then from the menu bar at the top of the screen, click on the "Edit" menu and choose "Enable Root User". Anyone can login as "root" with empty password after clicking on login button several times. A recently uncovered bug appears to allow anyone to login as an administrator merely by entering the username "root" and no password. You really shouldn't leave your Mac unattended at all until Apple fixes this, and you should shut off guest access for your device.
It is possible to mitigate against the flaw, however, by adding a password for the root user in the users and groups preferences pane. They could reset or change passwords, delete or add users and Apple IDs linked to the machine, and dip into other accounts on the system - essentially, they would get unfettered access to all the data that lives on the computer.
You can patch this problem right now by creating a root account manually and giving it a secure password.
The issue was made public by software developer Lemi Orhan Ergin, who demonstrated the flaw and reported it to Apple's tech support account. Then, click the "Join" button beside "Network Account Server" and a new panel will pop up.
Currently, there is no official fix from Apple regarding the issue.
Click the lock icon in Directory Utility's window and authenticate.
Once a password has been set for the "root" account, the flaw that allows a person to login as "root" with no password will no longer work.
So far as we can tell, you need access to a now logged in account in order to trigger it.