is a Linux command for substitute user and group rights while running a command within the Terminal Activity or a command console. In its default form, , it enables the root or admin or super user's permissions to modify the system (see user).
It gives you a login shell session that affects the commands that are available.
Various instructions will direct you to type( being a lower-case letter L, for login) or just . On some laptops, the next prompt will be "Password:". If this happens, and you do not have a root password, type instead.
Enter the Ctrl D key combination (or enter thecommand) to exit the root user session.
See reference wiki, http://man-wiki.net/index.php/1:su