what does the title 'the catcher in the rye' relate to the novel? i don't understand the song that Holden hears about it or how it relates to him.

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.