Here's what it says under one of the DB FAQs I found doing a search on post reminders:

Post Reminders:
This is probably one of the most under-utilized, yet most powerful features here at our site. Do you pop in here to check messages through the day? Does someone interrupt you when you are trying to compose a reply? Well the solution is easy. At the bottom of each post, you'll see a tiny "Remind Me!" icon, like this: . If you click this icon, a reminder will be stored for you to make a reply to this post later.
To return to the post, click the link right here in the Post Reminders section. Once you have posted a reply, the reminder will automatically disappear. If you change your mind, you can check the reminder and click "Delete" to remove them.