As an editor, I often tell my clients to add more to their story. Most of it are the “how” and “why”s of a story.
“How did they get from Point A to Point B?”
“Why did they go into that room without any reason?”
“Why are they spending hundreds of dollars in this scene when in the last scene they lamented having money troubles. Did they come into money? Are they bad with what little money they have? Were these two parts written at different times and need to be justified with each other?”
On the converse side of this is having too much information. There are a number of ways this can spell disaster in a story.
First, false leads. By providing too much information to your reader you are creating the expectation that the information will be important later. Your reader will now spend time trying to figure out how that information plays into the story, and they’ll feel the information was misleading if it doesn’t matter in the end (in the sense that the extra/unnecessary information you provided doesn’t change the story if it’s ultimately removed.
Second, fatigue. Writing, and by relation, reading, is a fine balance of growing more tired and countering that with excitement from the book. Having too much fluff in a book or story that isn’t pertinent to the story creates fatigue in the reader but doesn’t offer excitement as a cure.
Third, and last, it might give the impression of bad writing. Saying, writing, too much comes across as not knowing where to end the sentence or the story.
It’s far easier in a short story than a novel to remove unnecessary writing, things that change the story in no way whatsoever. In a novel, some writing that seems unnecessary on first glance can actually have color and flavor in it that builds the characters. It’s hard to know without having an editor or beta reader to look over your work and provide feedback.