Here is a description of each field of the dialog.
A radio button lets you select between a reboot or a shutdown. A shutdown performs the same operations as reboot except that it brings the machine to a complete endless stop. The only alternative left after that is to turn off the power or use the reset button to restart it. A reboot will end all of the processes, but once all of them are shut down, it will launch the startup sequence again.
On a multi-user system or a server, it may be wise to give people some time to finish their current task before shutting down the computer. If you just want to shutdown or reboot NOW, then enter 0 here. The number is the amount of minutes between the message broadcast and the effective shutdown.
Currently the shutdown feature of Linux can send a message (a small explanation) on terminals. It is expected that this message will be sent to users of other services (samba for one).
Place whatever you want to broadcast here, including nothing. It is simply written to the tty of every user currently logged in.