The closing chapter of the novel identifies the setting of the narration as a place - most likely a clinic or hospital -where the narrator, Holden Caulfield, is recovering from a mental breakdown. Leading up to this chapter, i.e. from Chapter 1 to 25, is the story of how he comes to be in this place.
It is also in the last chapter that we can identify the "you" Holden has been addressing throughout as someone who is not directly connected to the events of the narrative. Most likely, the "you" is the reader to whom the narrative is addressed. All the same, she/he is an ideal listener for Holden.
Very early in the first chapter, Holden sets the parameters (and terms) of his narrative in the following:
. . . I'm not going to tell you my whole goddam autobiography or anything. I'll just tell you about this madman stuff that happened to me around last Christmas just before I got pretty run-down and had to come out here and take it easy. (1994: 1)
Granted, a story about a few days in the life of a fictional character can hardly be called an autobiography. So what is it? It is important to keep in mind that what we call it will ultimately determine, or at the very least shape, how we read it.