The term for a story's underlying message is "theme." The theme conveys the central ideas or insights about life, society, or human nature that an author wishes to communicate. It is often expressed through the plot, characters, and settings and provides depth to the narrative. Themes can be explicit or implicit, and they encourage readers to think critically about the text. Common themes include love, conflict, redemption, and the struggle between good and evil. By exploring a story's theme, readers gain a deeper understanding of the characters' motivations and the broader implications of the plot.