The passage describes Curley's wife cutting off the sunlight when she enters the barn where George and Lennie are working. Her entrance makes the two men look up, suggesting she holds some power over them in the hierarchical ranch setting. The reference to her cutting off the "rectangle of sunshine" foreshadows that she will block George and Lennie's hopes and dreams of owning a farm together. Descriptions of the sunshine as warm and comforting contrasts with how her cutting it off changes the tone and leaves the men feeling cold, just as by the end of the novel their hopes are gone with the setting sun.