The system will automatically send this message to anyone trying to message you. The message will also show in your information if someone does a whois on you. It is often used to tell people that you are away from your computer. If you no longer wish to be seen as away, then simply type /away with no message and the away message will no longer be displayed.
Example:
/away I am away from my computer
/away
If you will be logged onto the chat network, but not actually reading text from it, then you should mark yourself as away. If you do not mark yourself as away then people may try to speak with you and be offended when you do not reply. You can also use it if you are staying active but do not want to reply to private messages. For example, some people will set their away message to something like "Please talk to me in the room, I am busy and not replying to private messages."