I remember from reading the novel many, many years ago that it has something to do with the Robert Burns poem Comin' Through the Rye, and the wiki entry for the song says this:

"The title of the book The Catcher in the Rye comes from the poem's name. Holden Caulfield, the protagonist, imagines children playing in a field of rye near the edge of a cliff, and catching them when they start to fall off."

And there's this from Sparknotes:

4.        . . . 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.
Explanation for Quotation 4 >>
This, the passage in which Holden reveals the source of the book's title, is perhaps the most famous in the book. It occurs in Chapter 22, after Holden has slipped quietly back into his apartment and is speaking with Phoebe. They talk, argue, and then reconcile, and Phoebe asks Holden what he wants to do with his life. Holden responds with this image, which reveals his fantasy of idealistic childhood and of his role as the protector of innocence. His response makes sense, given what we already know about Holden: he prefers to retreat into his own imaginary view of the world rather than deal with the complexities of the world around him. He has a cynical, oversimplified view of other people, and a large part of his fantasy world is based on the idea that children are simple and innocent while adults are superficial and hypocritical. The fact that he is having this conversation with Phoebe, a child who is anything but simple and innocent, reveals the oversimplification of his worldview. Holden himself realizes this to a degree when he acknowledges that his idea is "crazy," yet he cannot come up with anything more pragmatic; he has trouble seeing the world in any other way. His catcher in the rye fantasy reflects his innocence, his belief in pure, uncorrupted youth, and his desire to protect that spirit; on the other hand, it represents his extreme disconnection from reality and his naïve view of the world.

Cheers

LFF