This message from the system shutdown(1M) script informs you that the superuser is taking down the system.
Save all changes now or your work will be lost. Write out any files you were changing, send any email messages you were composing, and close your files.
For more information on shutting down the system, see the System Administration Guide, Volume 1. If you are using AnswerBook online documentation, "halting the system" is a good search string.