When exactly? In the entire novel?
Here are the basics:
In chapter 2, Slim is introduced as the unspoken leader of all the men on the ranch. His word is law and all of the other members of the community respect him, including the boss and his son Curley.
There is an accusation by Curley that Slim is sleeping with his wife but this proves unfounded.
Some of his actions include convincing Candy to let Carlson kill his dog. "That dog ain't no good."
He also helps George to better deal with Lennie and he is the only one to truly understand their relationship.
At the end of the novel, when George shoots Lennie, Slim replies "You hadda, I swear you hadda" as he puts his arm around George and comforts him.