Curley's wife has come into the men's bunkhouse uninvited. Once there, she starts trash talking Crooks, Candy, and Lennie. Normally, Crooks is very demure around Curley's wife; as the only African-American on the ranch, he knows he could get into serious trouble if he steps out of line. But on this occasion, Crooks angrily lashes out at Curley's wife, giving her a piece of his mind.
Curley's wife's not used to being talked to like that by someone she regards as a racial inferior. She puts Crooks back in his box, so to speak, and tells him to "keep his place," meaning that as a black man he needs to show respect for a white woman. More ominously, she makes a none too subtle threat to have him lynched if he should step out of line again:
I could get you strung up on a tree so easy it ain't even funny.