The message of the day is the text that's displayed to you when you log in. At the moment, mine was just an awesome Debian symbol, with the address of my maintainers. Whilst the ascii art was awesome, it didn't really help me.

You can change your motd by sudo editing /etc/motd. It's as simple as that really.

Once you change it, it'll be system wide, meaning every user will see that message when they log on. Keeping that in mind, it's a good way to get a message around to users without having to send tonnes of emails to each of them. For instance, you could put a "Server will be down for maintanance next Tuesday" message for everyone to see. If you're the only person using the box (like I am), then you can just putting motivational messages, or a todo list.