Email autoresponders work like a fax-on-demand system. Maybe you've seen the systems where you call from your fax machine and dial a certain code and you'll get back a document in your fax machine? Well, an email autoresponder works the same way. If you send an email to an autoresponder address you'll get back a prewritten message.
Until just recently, you could only do a one-time auto responder with one message coming back. But now you can set up a whole sequence of autoresponder messages going out on any day you choose.