The theme of a work is the "author's message" to the reader. It is seldom directly stated. Instead, the theme is something that a reader realizes when they finish a particular work. Usually the main character changes or learns something in a novel. If you look at how the character changes, you will begin to realize what the theme or author's message is.
The theme is different from the plot. The plot is all about the action. It concerns who did what to whom. The theme is much more about understanding some deep truth that the author wishes to convey to you the reader.