An Autoresponder is an email that you can set up to be a stand alone, or one of a series, that will be sent out at a set time after a person subscribes to your contact list.

For example you can set up an autoresponder  to be sent immediately (0 hours) after the person signs up to let them know about a promotion you are having for new users. 

You could then set up one to be sent 72 hours, or 3 days later see how they felt about your offer.