Puts you in mail mode. You can press return to view mail messages. s filename will save, or append the mail message you just read to the file filename. d will delete the current message and show the next message. h will show you the current list of messages. Entering the message number followed by pressing return will allow you to read that particular message. q will quit, or take you out of mail. For more information on mail, you can type ? while in mail mode. Typing man mail from the original prompt, which is not inside the mail system, is another method for obtaining more information on mail.
There are other mailing software packages available such as mh and xmh. For more information on those, view their man pages.