In chapter 22 of The Catcher in the Rye, Holden tells Phoebe that he does not want to be a lawyer or a scientist. Holden responds that he would not be a good scientist because he is no good at science. He declines the prospect of being a lawyer because all lawyers do is play golf, drink martinis, and make money. Obviously, neither of these careers suit Holden. Instead, Holden tells Phoebe what he would really like to do.
Anyway, I keep picturing all these little kids playing some game in this big field of rye and all. Thousands of little kids, and nobody's around--nobody big, I mean--except me. And I'm standing on the edge of some crazy cliff. What I have to do, I have to catch everybody if they start to go over the cliff--I mean if they're running and they don't look where they're going I have to come out from somewhere and catch them. That's all I'd do all day. I'd just be the catcher in the rye and all. I know it's crazy, but that's the only thing I'd really like to be.
The cliff, for Holden, would represent mistakes children make in their lives (whether simplistic or one where the chance becoming a phony (something Holden hates)). The catcher in the rye represents Holden himself. He wants to be the person who stands at the edge of the cliff insuring that the children do not run blindly, or at all, over the side. Essentially, Holden wants to be able to protect those younger than them, for it seems he still has hope for those like Phoebe. The reference to the catcher is used because it refers to the catcher's mitt (Allie's). Holden has always looked up to Allie and this reference shows the depth of his admiration.