In chapter 16 of The Catcher in the Rye, Holden Caulfield overhears a young boy singing,

If a body catch a body coming through the rye.

The line comes from a poem by the eighteenth-century Scottish poet Robert Burns. Holden is quite taken by these words, even though he doesn't quite know what they mean. What he does know, however, is that they immediately make him feel less depressed. This is because Holden is struck by the child's innocence. There's absolutely nothing “phony” about him, and for Holden to encounter such perceived authenticity in another human being is a rare privilege indeed.

Holden's overhearing of the boy singing forms the basis of a fantasy that he shares with his sister, Phoebe, in chapter 22. Holden imagines that there are thousands of children running around and playing a game in a big field of rye. He's standing on the edge of a cliff, and whenever any of the kids get too near to the edge, Holden catches them. Hence the title of the novel is The Catcher in the Rye.

That's the literal meaning of the book's title. But the symbolic meaning is that of protecting childhood innocence. It represents the deep-seated desire of Holden, who simply can't handle life in the “phony” adult world, to preserve the innocence of childhood, both in himself and others.