An autoresponder sends out a series of messages to its subscribers. It will send each message a set number of days apart, or a set number of days for each message after the subscriber joins the list.
For example, an autoresponder could send out each message 3 days apart, starting on the day the subscriber joined the list.
Here is another example. A subscriber joins the list and message number 1 is sent to him immediately. The next day, message 2 is sent to the subscriber. 3 days later another message is sent to the subscriber. The next message is sent 5 days after the last message. From this point on, a new message is sent each week.
This was just a simple explanation on how an autoresponder works. You can configure an autoresponder to send messages in many fashions and time frequencies. You could even configure an autoresponder to send the messages randomly and even start from the beginning after they have received all of the autoresponders messages.