Rebooting means booting the system again. This can be accomplished by first shutting it down completely, turning power off, and then turning it back on. A simpler way is to ask shutdown to reboot the system, instead of merely halting it. This is accomplished by using the -r option to shutdown, for example, by giving the command shutdown -r now.
Most Linux systems run shutdown -r now when ctrl-alt-del is pressed on the keyboard. This reboots the system. The action on ctrl-alt-del is configurable, however, and it might be better to allow for some delay before the reboot on a multiuser machine. Systems that are physically accessible to anyone might even be configured to do nothing when ctrl-alt-del is pressed.