“Remove everything that has no relevance to the story. If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it’s not going to be fired, it shouldn’t be hanging there.”
Chekov is urging the writer to be honest with the reader and is warning against extraneous detail.
A gun is a strong image, full of meaning. It has the potential for danger and death or the saving of a life. To draw it to the attention of the reader means that if nothing comes of it, readers can feel duped. Every detail must have purpose. If you give something significance you should follow through on it.