If an adult has every gotten on your case about uptalk, vocal fry, or saying “like” all the time, you get the point of Catcher in the Rye—you and Holden might say different things in different ways, but you both speak the same language: teenager. Holden’s style (which is the book’s style) is colloquial and slangy, sounding a lot more like a real seventeen-year-old talking straight to you than an accomplished adult author.
Some examples? He says things like "You'd have liked [Allie]" to give the illusion that he’s right there talking at you. He uses italics to make the words read with the same emphasis as spoken word ("He's my brother and all"). You'll hear him describe places and people all the time as "corny" or "phony." He'll tell us he's never waited anywhere so long in his "goddamn life. [He] swear[s]" (24. 97), or that he's sweating "like a bastard" (24.100).
The Catcher in the Rye, like Adventures of Huckleberry Finn before it, is one of few books to feature this language in the narration itself, not just in dialogue. At the time, this was both unusual and important—not just as a new literary style, but also as a way to study the vernacular of a particular time period. So, while the language doesn't seem all that offensive to us (PG, maybe), it raised a few more eyebrows in 1951.