Final answer:
User id 0 refers to the root user, who has full administrative privileges on Unix and Linux based systems.
Step-by-step explanation:
In the context of operating systems, particularly Unix and Linux based systems, user id 0 is the id of the root user. The root user is the superuser with full administrative privileges, allowing this user to have access to all commands and files. It is named 'root' because it is the user with the privileges to perform any action on the system, much like the roots of a tree being the foundation. It is important for the management of the system, but also represents a significant security risk if misused, hence why it's generally advised to use the root user sparingly and with caution.