In this scene Lennie is upset. He is upset because he has killed a little pup that slim gave him. He is upset because he knows that George will not let him tend the rabbits. That is the only thing that Lennie really cares about about is tending the rabbits.
When Lennie realizes that he has killed the poor pup he tries to cover it with hay until a woman wearing a red dress comes in and tries to talk to him. It's Curley's wife.
Curley's wife comes in and try to talk to Lennie but he won't budge. She keeps on trying to talk to him because she feels like she can't talk to anyone. Lennie knows he can't talk to her because he won't be able to tend the rabbits.
Curley's wife lets Lennie feel her hair because it's soft and he likes soft things. Lennie gets too aggressive with it cause her to panic. He tries to calm her down and ends up snapping her neck. She dies and he knows he is in trouble. He tries to cover her up with hay which doesn't work.