An element of plot which is important for understanding the theme of a story is setting.
A setting in a story refers to the place and time where a scene occurs. Setting helps in setting the mood of the character and their behavior. Further, it reflects the character's society in which they live and can sometimes play as a part of the story. In addition, setting also helps in providing the framework about what is being discussed.
Hence, what describes best the role of the setting in the story is setting is used to emphasize ideas and theme.