Subscribers sign up to receive emails from you, probably as a result of a free gift, a new product or service. To keep in contact with them you need to have an autoresponder in place to help you.
An autoresponder is a service which allows you to automatically send out emails to a group of people, or to several different groups of people in your niche.
The main purpose is to build your relationship over a period of time with all your subscribers and keep in regular contact with them. Your task is to build up a series of emails that will provide your list of subscribers with useful information, resources, products and services that they may find useful now and in the future.
Once you have set up your emails for your subscribers and chosen the frequency, they are sent out automatically for you by the autoresponder.
If you google autoresponder you will come up with a list of different ones, such as GetResponse, Aweber, Mailchimp and lots of others. I mention these ones only because I have used them.