The above editor is correct: the information you need is in chapter four of the novel, rather than in chapter two.
If you need evidence for your answer, it begins on page 75 of my edition. The entire chapter takes place in Crooks' room, which is adjacent to the barn and is entered through the barn. When Lennie appears and explains that he saw Crooks' light on, so he came in, Crooks says:
"Well, I got a right to have a light. You go on get outta my room. I ain't wanted in the bunk house, and you ain't wanted in my room."
"Why ain't you wanted?" Lennie asked.
"'Cause I'm black. They play cards in there, but I can't play because I'm black. They say I stink. Well, I tell you, you all of you stink to me."
Crooks isn't allowed in the bunk house, because of segregation. The races are kept apart. The reader assumes that the idea of Crooks stinking is a stereotypical idea the whites project upon him due to their prejudices. They don't want him in the bunk house because he is black.
The stable buck is a man named Crooks. He is given his own room because of prejudice. What I mean by that is that he is a black man and that is why he gets his own room.
In those days, blacks and whites simply did not mix. White people would not have felt at all comfortable living around blacks -- they would have felt like that was beneath them.
You have this labelled with "Chapter 2" which is sort of strange because there is very little reference made to Crooks at all in this chapter. The only possible evidence is that we are told that the boss gives Crooks "hell" apparently because he's black.