A summary ep that reviews the characters and important events of the preceding episodes, and gives important information that will help you enjoy the climax 10 times as much!
This is actually EPISODE # 20 on the regular show. Even though AP has the regular show as 24 total episodes (which is right), this unofficial special (or this de-facto episode #20) has increased the episode count on most anime watching websites to 25. So you might be deceived in thinking that there are 25 episodes in the regular show. Again, if you've already watched the regular show (& its 25 episodes on most anime viewing websites) then you've probably watched this as well (without realizing it was the special episode). This special (or de-facto episode #20) is just a review of the past episodes. I dropped it halfway but I'm pretty there were no new scenes in this one. It was just rehearsing old scenes from past episodes with some narration. You can watch it if you didn't understand some stuffs otherwise I suggest you skip it to save yourself the 25 minutes (you might spend watching this).