What term describes the central message or insight revealed in a work of literature?

The central message or insight is termed theme in literature. This is the main message of a literary work and the theme may be revealed either directly or indirectly.  Themes can also be minor or major themes, the major one's that are repeated, which makes them the main themes of the work while the minor ones are those which refer to ideas that appear briefly.

The central message or insight revealed in a work of literature is often described as a Theme. The Theme is often confused with the Tone. Theme is the message, while Tone is the author's attitude toward the subject of his or her writing.