What happened in the barn between Lennie and Curley's wife?
Well, what eventually happened is that Lennie killed her. But let's look at what happened leading up to that.
Lennie is in the barn, hanging out with a dead puppy. Curley's wife comes in and starts talking to him. They talk about his and George's dream of the farm. She talks about how she could have been in movies if it weren't for Curley. She talks about her frustrations and her ambitions. She criticizes Lennie some for being afraid to do things George tells him not to do.
Then comes the fatal part. He talks about liking to touch soft things. She has him touch her hair. She starts to get mad because he's going to "muss" her hair. She tries to pull away and he tries to stop her from struggling for fear George will be mad at him. And he ends up breaking her neck.