What does the barn symbolize in Steinbeck's Of Mice and Men?

The barn symbolizes, or represents, animalism. While it is a safe place for the animals, and a place where the people feel that they are less inhibited, it is also a place in which characters display behavior that is more indicative of their fundamental nature. 

In Chapter 4 of Mice and Men, since George has gone to town with the other men, Lennie comes to the barn to pet his puppy; however, he notices the light in Crooks's room, so he stops. He is met with hostility by Crooks, a defensive hostility issued from years of rejection:

"You go on get outta my room. I ain't wanted in the bunk house, and you ain't wanted in my room....Don't come in a place where you're not wanted."

Once Crooks realizes that Lennie is simple-minded, his hostility dissipates; nevertheless, because of naturalistic impulses, he cruelly toys with Lennie, as he enjoys his advantage over this white man. He asks,

"...s'pose George went into town tonight and you never heard of him no more....They'll tie ya up with a collar, like a dog."

Suddenly in his fear, Lennie stands over Crooks and contradicts him. Fearful of the physicality of this big man, Crooks tells him to sit down. Then he explains,

"S'pose you didn't have nobody? ...S'pose you didn't have nobody....S'pose you had to sit out here and read books?" 

Then, Candy enters and Crooks invites the swamper in, although the man has never been in his room before. Candy speaks of the dream of owning a place, but Crooks cynically derides him, "...you'll won't get no land. You'll be the swamper here till they take you out in a box."

Hearing the men, Curley's wife appears, rubbing the nails of one hand with the thumb and forefinger of her other hand, as though she were sharpening claws like a cat. After looking at all of the men, she says, "They left all the weak ones here....I know where they all went." Then, she berates the three men, telling them that they are all afraid of each other. "Ever'one of you's scared the rest is goin' to get something on you."

Continuing her insults, Curley's wife figuratively digs her claws into the men because they will not tell her how her husband hurt his hand. Then, she notices the men's hostility as she looks "from one face to another, and they were all closed against her." But, before she leaves, she turns to Lennie and tells him that she is glad that he "bust up Curley a little bit," adding that she would like to "bust him myself."

In the next chapter, death enters the barn as Lennie's big "paws" first kill the puppy, and then in fear they try to keep Curley's wife quiet when she cries out.

She struggled violently under his hands....her eyes were wild with terror. He shook her then...and her body flopped like a fish. And then she was still, for Lennie had broken her neck.
He pawed up the hay until it partly covered her.... Curley's wife lay with a half-covering of yellow hay.

The calm of the barn for the animals is now gone. "The horses stamped and snorted...and they clashed the chains of their halters" as they sense the death that disturbs their environment.