The gun above the mantle is a concept in writing which denotes the value of the words you write. It can be argued that the letter written by Anton Chekhov to a friend is one of the most important rules in writing: That everything written into your narrative be irreplaceable, and that you should leave out all else.
Thus, the rule reads, "If in the first act you have hung a pistol on the wall, then in the following one it should be fired."
This implies that every element of your narrative be as important as your characters. The reason for this is twofold. First, this removes fluff from your story, as nobody enjoys reading filler.
The second reason is to denote significance. You have only a finite number of pages between the front cover of your book and the end.
Never shall a single page be able to be deemed "unimportant."