A prequel is set before the original story, explaining background details or characters' histories. After watching the movie, I read the prequel to learn about the hero's past.
Pro Tip 2/3
Not Always First
Prequels are released after the original series, despite the story timeline being earlier. The second book published in the series is actually a prequel to the first book.
Pro Tip 3/3
Understanding Characters
Prequels can give you insight into why characters act the way they do in the main series. The prequel shows why the villain turned evil, adding depth to the story.