The second category on the Webmin tab bar is System. Clicking it will allow you to edit and/or monitor such system features as: Bootup and Shutdown behaviour, Disk Quotas, Filesystems, Manual Pages, NFS Exports, Processes, Cron Jobs, System Logs, and more. The figure below shows the options available on a Linux system (specifically a Swell Tsunami web caching appliance running Red Hat Linux).
Clicking bootup and shutdown brings you to a page of bootup options. In the case of a Red Hat system, it provides access to all of the initscripts found in /etc/rc.d/init.d. Clicking on any of the script names will provide the ability to edit, start, stop, and delete the initscript. Usually, each initscript provides functions to start, stop, and restart system services such as Sendmail, named, and Apache, as well as perform basic system initializations such as setting up network devices and routing tables. An easy way to quickly add a new service or command to the system startup routine, if it does not have an initscript, is to add it in /etc/rc.d/rc.local.
Also on this page are the Reboot and Shutdown buttons. They do just what you would assume, after a confirmation screen.