Well, Pop Reel is when one of your submissions goes into the Whats Hot section.  Then it appears on the main screen. Many people write POP REEL in the title of their submission if it gets popular. I have had POP REELs before and I do the same thing. For a submission to become POP REEL it has to be good and get a lot of views. See my other blog about how to make popular blogs. Otherwise, good luck at trying to make POP REELs!
If you want some advice about going POP REEL, you could look at the submissions in the Whats Hot section or my blog or another about making popular submissions. If you're not sure if your submission is POP REEL, look through the Whats Hot section and see if it's there. 
Remember, if your submission doesn't go POP REEL that doesn't mean it's bad! It just means nobody was probably interested by the title and title picture or the topic itself. 
Your blog will probably be POP REEL if it has more than 60 views. In some cases blogs with 35 views are POP REEL. Skins usually require more views to go POP REEL.

Thanks for reading and good luck making POP REELs!