Let's say you've gone on holiday, and you want to inform those who write to you that you won't be able to get back to them for a few days. An auto-responder would be helpful in this case.
Essentially, you can write a message that will automatically be sent to anyone trying to contact you via email. You can set up 'auto-responders' very easily.
1) Simply log into your cPanel (www.yourdomain.com/cpanel)
2) Click on 'mail'.
3) Click on 'auto-responders'.
4) Type in what email address this auto-responder will apply to, and create your message.
5) When you get back from your holiday, just do steps 1-3 again, then delete the auto-responder.