In this chapter/section taking place in Crooks' room off the stables, the commonalities and differences between Lennie, Crooks, Candy and Curley's wife are examined.
These characters are all left behind when the other ranch hands go to town. This fact is important as it signifies these characters' identities as social outcasts. For various reasons, these four figures are not invited into town to share some social time with the others. Thus, one of the major commonalities shared by these characters is the status of outsider/outcast.
The differences between the characters are stark. Crooks repeatedly tells Lennie that he should not be in his room. Candy repeats this message and so does George. This fact is the upshot of Crooks' being non-white in terms of skin color.
Curley's wife is also shown to be quite different from the men and set apart firmly. We see this in the way the three men band together to oppose her - opposing her presence in the room as well as her opinions on the prospects of them getting a ranch of their own. She responds by threatening Crooks, demonstrating what little power she has (derived from a social code that places Crooks at the bottom of the social order).
Each of these figures craves company and would like to overcome the social strictures that enforce their isolation. Yet, these strictures are not easily overcome. This is one of the messages of the chapter.