Prose craft
Theme
A story's major topic, subject or message, such as love, death or betrayal: what the work is about beneath its plot.
A theme is "any major topic, subject, or message within a story," the definition used across narratology, literary criticism and film studies. It is usually nameable in a single abstract noun, love, death, betrayal or parenthood, or a phrase such as coming of age, and it sits among the basic components of fiction alongside plot, character, setting and style. Longer works typically carry several themes, usually implied rather than stated outright.
The article's useful distinction splits the thematic concept, what readers "think the work is about," from the thematic statement, what "the work says about the subject": loneliness is the concept in Of Mice and Men, and what the novel argues about loneliness is the statement. For serial writers theme is the rudder: plot decides what happens next, theme decides what the events mean, which keeps hundreds of chapters from feeling episodic. Theme usually becomes visible through the character arc, and a tradition like the bildungsroman is a theme promoted into a genre.