The phrase "catcher in the rye" is first mentioned in Chapter 16 when a youth is singing the song "Comin' Thro' the Rye" whilst wlaking in the street instead of walking on the sidewalk. Then, in Chapter 22, Holden responds to Phoebe's question about what he wants to do in his life by saying he wants to be a "catcher in the rye," where he imagines a field of rye on a cliff full of children playing. He wants to protect those children from falling off the cliff:
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.
What is important about this quote is the way that it reveals the fantastical innocence of Holden and how he envisages childhood as an Edenic-like existence that needs to be protected and safeguarded. Some incidents that show Holden would be good at this task would be the way that he repeated refers to adults and their world as being "phony" whereas he views the world of children as being innocent and simple. You could consider the way that he spends a great deal of time and effort trying to erase a swear word that has been scrawled on the museum to protect children from seeing it and therefore being corrupted. He is, through this simple act, playing the part that he desires to fulfil, protecting children from being corrupted.