motd - message of the day
The abbreviation "motd" stands for "message of the day", and this file has been traditionally used for exactly that (it requires much less disk space than mail to all users).
On Debian GNU/Linux this file is a symbolic link pointing to /var/run/motd. The contents of this file are regenerated upon every system boot based on the contents of /etc/motd.tail.
This page is part of release 3.35 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man-pages/.