This message means the system is going down immediately and it's too late to save any changes.
This message is often preceded by messages telling you that the system is going down in 15 minutes, 10 minutes, and so on. When you see these initial broadcast shutdown messages, save all your work, send any email you're working on, and close your files. Fortunately vi(1) sessions are automatically saved for later recovery, but many other applications have no crash protection mechanism. Data loss is likely.
For more information on shutting down the system, see the System Administration Guide, Volume I. If you are using the AnswerBook, "halting the system" is a good search string.