Holden hears a little boy singing a song while walking along the streets of New York. The song/poem has a line about "a body meets a body coming through the rye." The boy says (or Holden hears) "a body catch a body coming through the rye."
Later Holden imagines himself as a "catcher in the rye." He thinks of a bunch of little kids playing in a field at the edge of a cliff. Holden would be at the side of the cliff, and if any child was falling off, he would catch him.
This is Holden's fantasy about being able to save a child, save innoncence. This is shown in many other places in the novel. For example, he wants to erase swear words off the walls at Phoebe's school. He wants to protect the children of the school from seeing such offensive language.
At the end of the novel though, Holden realizes that he can't be "the catcher in the rye." He can't save everyone. Sometimes children have to learn for themselves, learn the hard way. This is evidenced when he worries but doesn't act as Phoebe is reaching for the brass ring. He lets her reach dangerously off her carousel horse.
i meant how not what