Don't worry about it. It just means one of the background Unix maintenance programs sent some kind of message to the root account inbox. All it would ever be is just some status report message or an error report that probably wouldn't make any sense to you, and is of little to no consequence even if it did.
If you're really that curious, just type mail at the command line. "man mail" to learn how to use the command line Unix mail program (press q to quit). You will be extremely underwhelmed however, I'll just tell you that now.
When I log into terminal I get the following message. "Last login: Mon Jun 6 09:26:26 on ttys000
You have mail." Does anybody know what this is and how do you access this mail. My only mail program that I use which is Apple's Mail does not have any unread mail.