George learns very quickly that Curley is the boss's son and a bully. He is short and has been a lightweight boxer. He doesn't like men who are bigger than he is, like Lennie.

When Curley first meets Lennie, he bullies him because Lennie lets George talk for him. (George warned Lennie beforehand to keep his mouth shut.) Curley orders Lennie to talk for himself. Curley seems to be spoiling for a fight with Lennie.

After Curley leaves the bunkhouse, George says he's scared of the situation they are in because of Curley. He says they will get fired if Lennie gets into a fight of any sort with Curley. He advises Lennie very strongly to stay away from Curley as much as possible:

Look, Lennie. You try to keep away from him, will you? Don’t never speak to him. If he comes in here you move clear to the other side of the room.

Lennie agree to do this.

In a nutshell, he tells him to stay away from his wife, only known in the novel as Curley's wife.  Curley is excessively protective of his wife and suspicious and jealous of anyone who tries to interact with her in any way.  When Lennie expresses to George that he thinks Curley's wife is "pretty", George realizes that Lennie may get in trouble by showing any interest at all in Curley's wife, so he wants to assure that Lennie doesn't interact with her.  In addition, George feels it is best that he stays out of Curley's way because Lennie often tends to unintentionally get in trouble whenever he opens his mouth.