What does importance to the plot means?

You know what a plot is, right? It's a story line. Any facts which are important to the plot are necessary to know in order to understand what is going on in the story. The easiest examples are jokes. Have you ever told a joke where you got to your punchline and, instead of the expected laughter, you got puzzled looks? You left out something important to the plot. On the other hand, you have probably been annoyed by people who try to tell jokes and who get all hung up on details which are not important to the plot of the joke. Then you hear something like this: "A priest, a rabbi and a minister go into a bar. The minister is a Methodist, I think, although he might be Presbyterian. I don't think he was a Baptist, but he might have been from one of those churches . . ." It is about now that you wish the guy knew what importance to the plot means.