It is what is the underlying idea or meaning of a text. We are used to being asked after having read a book, or a story or watched a film, "What was it about?" "What is it like?" The answer to these questions is typically the theme. A narrative could have a plot and/ or subplots but what does the writer wish to convey using all of it and other literary devices? That central element is the theme. Now, theme is present in both prose and verse, in still images, paintings, lyrics, music and song, too. It is what is present throughout a piece of work, and in this particular case — literary work.
Love is a theme, so is revenge. Redemption, good vs evil, courage and so on, too, are some of the main themes. To consider an example: In Shakespeare's play Macbeth, good vs evil is the main theme within which ambition getting the better of one is the moral theme.