Explanation
The pam_nologin module is used to prevent users from logging into the system when the /etc/nologin file exists. The file can contain a message that is displayed to the user before the login is denied. The only exception is the root user, who can always log in regardless of the existence of the /etc/nologin file. This method is useful for system maintenance or emergency situations when normal users should not access the system. References: LPIC-2 exam 201 topics, pam_nologin man page