Follow up messages allow you to set predefined emails that can be sent to your subscribers at certain intervals.
For example, you can send the first follow up email of a list to a new subscriber one day after he / she subscribes to it. A second follow up email can then be sent 4 days after that first email. Finally, a third email can be sent 3 days after the second email, and so on. This allows you to create a relationship with your subscriber and let him / her know about your products / services.