A theme is a central idea in a work of literature. It's everywhere (cue creepy music), but it's never explicitly stated. For example, you might say "This book is about death," but the author probably wouldn't—at least not in the book itself. So death is a theme.
Themes help us reflect on big, hulking, abstract ideas like love, youth, progress, and religion. Once we've got the themes under our belt, it's time to figure out what the author is saying about that theme. The theme is the abstract idea, but the author probably has an opinion on it, right? And all the other aspects of a book—from the characters to the style to the plot—can help us pinpoint just what that opinion is.
There should be more than one theme in any given book—Shmoop usually suggests at least 8 or 10. Think you can find more? We do, too.