At the end of this chapter, George is worried about two people. They are Curley and Curley's wife.
He is worried about Curley because he is such a bad tempered man. And he is worried about Curley because Curley clearly dislikes Lennie. George worries that something bad will happen between the two of them.
He is worried about Curley's wife because she is always coming around and sort of making like she's coming on to the workers. It's like she's using them to make her husband mad.
As it turns out, he had good reason to be worried, because they both contribute to Lennie's death in their own ways.