A single plot, otherwise known as ‘the major plot’, generally isn’t enough to sustain a novel. Even if the major plot is strong, it will tend to fizzle out if not supported by one or more subplots.
Subplots perform an important role when it comes to pacing the novel. Interspersing the major plot with a subplot slows down the novel and increases tension. When you want to make the reader wait to find out what happens next in the major plot, you can simply move to the subplot.
The subplot must relate to the major plot in a critical way. While this relationship may not be obvious at the start of the novel, it must be apparent by the end. Subplots aren’t merely digressions into other, unrelated stories. They must be linked in some way to the major plot. The best subplots are those that are also thematically related to the major plot, perhaps highlighting another aspect of the theme, or approaching it from a different angle.
Subplots usually tell the stories of the secondary characters. Like the major plot, each subplot must be a complete story in itself, with a beginning, middle and end and an arc of its own.
When recognizing that their novel needs further development, novice writers sometimes make the mistake of adding more characters and too many subplots. This results in too many under-developed stories, and weakens rather than strengthens the novel as a whole.
It is therefore best to limit your subplots to two or three, and don’t add characters just to flesh out your story. Rather, develop your characters and their stories, and always make sure your subplots complement your major story.