what is system root?

In Finder Help, search on "Root" and click on "Enabling the root user".
This will give you an idea of what's going on, I think.

charlie

It depends on whether you mean the 'root' user account, also know as the "Super User". Logins to this account are disabled, but the account exists, as many of the privileged background processes are owned by the root account. NOTE: This is NOT the same as an "Admin" user.

Or do you mean the file system 'root' directory, which on a Unix system is represented by /. In the Finder, it is generally the "Devices" item with your boot disk name, such as "Macintosh HD", unless you renamed it.