Here is exactly what he said and what it was about...in the New York Times itself:
Donald Trump told the New York Times’ editorial board that his go-to applause line for sleepy crowds is about building a wall on the U.S.-Mexico border.
“You know, if it gets a little boring, if I see people starting to sort of, maybe thinking about leaving, I can sort of tell the audience, I just say, ‘We will build the wall!' and they go nuts.”