We covered the "Harry Potter" part of the title already (check out Harry's Character Analysis) so let's talk about the "Prisoner of Azkaban" part.

First of all, what the heck is an Azkaban? A mind-control spell? A new Muppet? If you were new to the Potterverse, you'd learn here that Azkaban is the Arkham Asylum of the Harry Potter world. All the baddest criminals are kept there. It's supposed to be impenetrable, but it isn't. Someone gets out, and that someone is the titular "Prisoner of Azkaban."

The prisoner is Sirius Black, and the title needs to be vague about his real role. Simply calling him the prisoner of Azkaban is 100% accurate without being a spoiler. None of these titles would work.

Harry Potter and His Godfather
Harry Potter and His Dogfather
Harry Potter and His Parents' Betrayer
Harry Potter and Gary Oldman's Prison Tats
Harry Potter and Taco Night

Those titles would either be lies or give away too much of the plot. "Prisoner of Azkaban" allows the story to be mysterious, while not betraying the trust of the viewer. But really, guys, when is taco night at Hogwarts?