holden tells phoebe that he wants to be a "catcher in the rye": what does he mean by this?

Finally Holden confesses to Phoebe that he would like to be a catcher in the rye: he pictures a lot of children playing in a big field of rye around the edge of a cliff. Holden imagines that he would catch them if they started to go over the cliff. Putting Phoebe back to bed, Holden decides to call up Mr. Antolini, a former teacher at Elkton Hills who now teaches English at NYU.