At the beginning of the chapter, Holden is feeling lost and alone. He is thinking about how much he hates pretty much everything about his life. He's also thinking about how he doesn't want to grow up. He is in the park and feels like he is watching everything from a distance, like he is not really part of it. It's a lonely feeling.
But then he sees the carousel, and everything changes. He runs over and gets on a horse, and for a few minutes, he is completely happy. He is not thinking about anything else, just enjoying the moment.
The carousel scene is important because it shows that Holden is not as cynical and jaded as he tries to make out. He is capable of feeling joy and happiness, even in the midst of all his pain and confusion. It also shows that Holden is still a child in many ways, and that he is still trying to find his place in the world.