This is generally used as the broadest topic of a post. If a post is a "reminder", that often means it's repeating something that you may have heard, but that bears repeating. In the age of information overload, it is not just the quantity of information, but the quality. And not JUST the quality, but the frequency with which certain topics are covered, which directs our attention towards certain things. Any centrally controlled social media outlet can choose a topic, and tell people what they're talking about, what's "trending", and turn people heads whatever way they want. They can make us see something and interpret it their way, by making us think that's just the "crowd", or they can misdirect our attention while other stuff is going on that they don't want you to see. So we need "reminders" of our own to keep ourselves on track.