In The Catcher in the Rye, what is meant by “the fall” that Mr. Antolini thinks Holden will have?

In J.D. Salinger's novel, Catcher in the
Rye, Holden has sought out Mr. Antolini, a former teacher, to talk to. He has
flunked out of school (again) and seems to be searching for direction. He has been
unable to find it at school. The teachers "teach" but do not become involved enough in
their students' lives to be considered "mentors." The fact that Holden has not applied
himself doesn't help, either— he has been "staggering" around, achieving nothing, and no
one has been concerned enough to provide him with any guidance or
support.

When Holden arrives at Mr. Antolini's apartment,
Mr. Antolini tells Holden that he has recently had lunch with Holden's father, and that
his dad is worried about Holden. Antolini says:

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I have a feeling that you're riding for some
terrible, terrible fall. But I don't honestly know what
kind.

Antolini seems
concerned that Holden will end up being someone who hates every
thing and every one. He certainly has little
patience with people, even those he seems to like, who become
unbearably annoying to him.

Mr. Antolini speaks again about
the fall he fears Holden will take. He indicates that it will occur because Holden will
fail to find something in life that he thinks should be there and
that when he "falls" there will be no way to even judge how far he
falls, figuratively speaking. He is afraid that Holden will become disenchanted and just
give up: quit—before he gives himself a chance to
succeed.

This
fall I think you're riding for—it's a special kind of fall, a horrible kind. The man
falling isn't permitted to feel or hear himself hit the bottom. He just keeps falling.
The whole arrangement's designed for men who, at some time or other in their lives, were
looking for something their own environment couldn't supply them with. Or they thought
their own environment couldn't supply them with. So they gave up looking. They gave it
up before they ever really even got
started...

Antolini tells him
that he envisions Holden dying for some cause that has no
value:

…I can
very clearly see you dying nobly, one way or another, for some highly unworthy
cause.

Antolini gets up and
and writes something down for Holden. He gives it to him—a paper with important
information that is not written by a poet but by a psychoanalyst. Antolini refers to the
author, Wilhelm Stekel:

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Here's what he said: "The mark of the immature
man is that he wants to die nobly for a cause, while the mark of the mature man is that
he wants to live humbly for
one."

Antolini continues by
telling Holden that he needs to find some direction for his life ("where you want to
go") if he wants to avoid "the fall"—failure; getting lost. Once
Holden makes up his mind, Antolini tells him that he will have to get to work right away
and, whether he likes it or not, he'll have to
return to school. Perhaps he is suggesting that when Holden grows
up a little more, he won't blame his teachers or his dislike of them
or his classes for his failures in school but will ignore them for
the sake of learning, because Holden is a lover of knowledge at
heart.

"The fall," then, is that Holden won't care about
anything enough to really try, and will amount to nothing. However, he can
avoid "the fall" if he grows up a little, finds what is important
to him in his life—regardless of the actions of others—and applies himself fully, with
intelligence and knowledge.