A theme is a universal idea, lesson or message explored throughout a work of literature. One key characteristic of literary themes is their universality, which is to say that themes are ideas that not only apply to the specific characters and events of a book or play but also express broader truths about human experience that readers can apply to their own lives.
All works of literature have themes. The same work can have multiple themes and many different works explore the same, or different themes.
Themes are also sometimes divided into thematic concepts and thematic statements. A work’s thematic concept is the broader topic it touches upon (love, forgiveness, pain, etc) while its thematic statement is what the work says about that topic.
Themes are almost never stated explicitly. Oftentimes, you can identify a work’s themes by looking for a repeating symbol, motif or phrase that appears again and again throughout a store since it often signals a recurring concept or idea.