"Peyton Farquhar was dead; his body, with a broken neck, swung gently from side to side beneath the timbers of the Owl Creek bridge."
This means that the main character, Peyton Farquhar, is dead and hanging from the bridge. It's a stark reminder that his attempt to escape from reality into a dream or fantasy didn't change the fact that he's actually dead. So, the theme is that fantasy can't change reality.