Holden is eating breakfast at a little sandwich bar at the train station when a couple of nuns come in and sit down next to him. The nuns are carrying suitcases, and it seems to Holden as if they're moving to another convent and are waiting for their train.
In any case, they don't appear to know what to do with their suitcases, so Holden gives them a hand. Holden also helps one of the nuns by picking up a collection basket that she accidentally drops. For good measure, he generously donates ten dollars to the charity for which the nuns are collecting.
What this episode illustrates is that Holden is instinctively drawn to people he perceives as being genuine. In other words, he likes those who are not phony. One reason why he likes the nuns is that he can't imagine them going anywhere swanky for lunch. He compares them favorably in this regard to Sally Hayes's mother, who, according to Holden, is a real phony.
Holden is sure that if Sally Hayes's mother were collecting for charity like the nuns she'd soon get bored, hand in her basket, and go someplace swanky for lunch. The nuns aren't like that; they're not phonies, and that's why Holden likes them.