The question is a little vague, but it hits at one of the most poignant elements in Salinger's Catcher in the Rye. Holden Caulfield is most clearly distressed by the death of his brother, Allie, who died of disease recently. Holden apparently was so distraught at this physical death that he broke his hand punching a car and was hospitalized, missing the funeral. The fact that an innocent and lovable young person could die is a thread Salinger returns to with increasing focus as the novel progresses. The loss of innocence and the inevitability of losing people haunt Holden, as the reader sees with increasing intensity.
These less tangible losses, or deaths (of innocence, friendship, belonging, and talent) punctuate the physical losses. For instance, Holden begins the novel struck by the minor illness of his history teacher at Pencey. Seeing the bodily fragility in age and illness, Holden recoils from Mr. Spencer and is unable to attend to the kindly advice he offers. Midway through the novel, Holden seems obsessed with sexuality—his own and that of a girl named Jane whom he likes. Being ushered into adulthood through sexuality seems traumatizing to Holden, who seems to want to make this leap but is also fearful of what that will mean for him. His series of encounters as he wanders Manhattan—with older women from Seattle, in conversation with former friends, with Maurice and the prostitute, and even the profanity he vainly seeks to erase on the walls of his sister Phoebe's school—show the intense pressure Holden puts on himself as he seeks to understand the adult world.
Late in the novel, Holden seeks comfort from Phoebe, who is still prepubescent. Taking her to Central Park to skate, he places himself in the locale of another motif—the ducks in Central Park and where they go in the winter. Do they die, he wonders, or just go away temporarily? With Phoebe, Holden seems to be able to relax, and it is in Central Park that he has an epiphany. Early in the novel, Holden reveals that he bought a hunting cap on his ill-fated trip to New York with the fencing team. He wears this cap throughout his long weekend as something of a helmet, or protection from the harsh reality of the tangible world. In fact, Salinger may be playing on Holden's last name, as the word "caul" is associated with a protective membrane on some newborns that must be removed at birth. The helmet—a hunting cap and therefore inappropriate to Holden, who is concerned with death and the ducks—must also be removed in order for Holden to progress in his growth. When he gives Phoebe the hat, he is making those first steps. Similarly, when he watches Phoebe on the carousel, he imagines wanting to catch her and all the children who may fall over the cliff (in his misremembering of the Robert Burns poem from which the novel is named). Accepting the fact of inevitable change, maturation, and even death defines Holden's necessary accomplishment in his story.