Have you ever thought about a book or a movie that you really liked, and why you liked it? Maybe the characters drew you to them and held your attention until they reached their goal. Maybe the setting just absolutely fascinated you. Or maybe you enjoyed the book or movie because there was so much going on and you had to weave your way through it all to find and enjoy the ending.
The really good books and really good movies have more than the main plot line to follow. They usually have at least one subplot line, often several.
To learn more about subplots, I have added my article on Subplots: How and When to Use Them to the Writing Tips on this blog and to my Writing Tools on my website.