How is the Catcher in the Rye a frame narrative?

Well first of all, think of a picture that is placed in a picture frame. The main focus when you look at it is the actual picture itself, but you can still see that it's not just standing there on its own, but has a frame around it.

In The Catcher in the Rye, we are introduced to the story in the first chapter by Holden who informs the reader that he will be narrating a story that already happened to him "last Christmas". Then he goes on to tell this story, and in the last chapter of it he returns us to the present where he informs us that "That's all I'm going to tell you about."

So we are informed at the beginning that this story has already happened in the past and we are being told about it in retrospect, and we are also reminded of it at the end. This makes this a frame narrative as it is basically a 'story within a story', a story that the narrator of this book relates to the readers.