Holden often uses the term 'Phony' to describe people and their actions. What does he mean?

Holden labels almost everyone a “phony,” excepting Phoebe, Allie, and himself. In Holden's eyes, a “phony” is someone who embraces the world’s mundane demands and tries to make something out of nothing—that is, just about everyone who studies in school or who puts on airs in order to do a job or achieve a goal. The fact that no one is acknowledging how trivial and fleeting life is, compared with the grand things we tell one another about reality—how difficult it is to truly love and share oneself with people knowing that all, like Allie, will eventually die—causes him to burn with frustration, even rage and label most people "phony."