Novels and short stories contain an underlying theme or multiple themes. These themes convey a message to the reader. A theme is usually expressed by good writers in a subtle manner so as to not preach to the reader or hit them over the head so hard that it distracts from the story's entertainment value.
Here are some great articles and resources about themes so can learn more about them and develop your own. You can find more resources for creating fiction in our fiction writing links section.
Children's Book Themes
Resources to help you discover many common themes found in children's literature.
Common Themes in Literature
The Writer's Academy from Penguin Random House describes some of the most common themes in literature including crime doesn't pay, love conquers all and war.
Finding Your Themes by Holly Lisle
Author Holly Lisle has a helpful article about finding your themes that includes the warning that if you choose to work with safe themes you'll write safe books.
How To Choose and Build a Powerful Theme for Your Story
This article from Well-Storied explains theme, how to identify your novel's thematic statement and how to integrate theme into your story.
Huge List of Common Themes
Literary Devices has a huge list of themes such as power and corruption, war, temptation and destruction, facing reality and good versus bad.
Leitwortstil and Thematic Patterning
Leitwortstil and Thematic Patterning are two techniques writers can use to express themes in fiction.
Purdue OWL on Theme
The helpful Purdue OWL calls theme the "meaning" of a story and says it is sometimes a lost art.
Twenty-Five Things Writers Should Know About Theme
Chuck Wendig provides 25 different things writers should know about theme including that it is a slippery business and it is not motif or a logline.