Post-purchase emails allow you to email your customers after their purchase has been completed. The email is sent to visitors whose last purchase was exactly X amount of hours ago. For example, say you want to send an email to visitors that haven't made a purchase in 3 months. For that purpose you will set the schedule to 2160 hours (90 days x 24 hours). Each hour the system will look at visitors whose last purchase has just completed 2160 hours and will send them the email.
Note that visitors whose last purchase was over 2160 hours before the rule was activated will not receive the email at all.
Also note that only the last purchase is taken into account; so if a visitor makes another purchase after 2000 hours then his counter will be reset and that new purchase will now be used as the reference time to determine if he is eligible for a post purchase email or not.