It’s the central concept that evokes a universal human experience and can be stated in one word or a short phrase (for example, “love,” “death,” or “coming of age”). The theme may never be stated explicitly, but it is exemplified by the film’s plot, dialogue, cinematography, and music (like the telling Batman theme song).
One of my favorite film themes definitely has to be ‘Freedom vs. Security’ in Captain America: The Winter Soldier. This is actually explored a lot in the MCU, but this film really hammers it home. Would you have a gun held to your head 24/7 if it means peace for everyone else? It’s a really complex message that a lot of other superhero movies don’t have the guts to explore.