You've probably heard this referred to as description. You have to describe enough of the trail of your story for it to:
- make sense to the reader
- keep the reader's interest
- fit your story's point of view (POV)
It's therefore handy to attack the setting (the description) as its own round in editing, particularly if you know it's one of your weak points.
What is setting? Setting is your world (where the entire story's set) and its locations (where each scene occurs).
Let's start with #1:
Your setting has to make sense to the reader.
The setting should make sense to you, the writer. It's your responsibility to convey enough of the setting so it makes sense to the reader, too—and in the proper order.
For example, Evonalé in A Fistful of Fire can produce purple fire with her magic. Since that's not the usual color that's associated with fire in the real world, I had to make sure that the fire's color was mentioned immediately the first time she did it. I couldn't wait until the end of the scene and have it as a punch line. At that point, it would've confused the reader.
For an example I didn't write, take The Emperor's Edge by Lindsay Buroker. The capital city, where most of the stories take place, is nicknamed "Stumps" because centuries before, a reputably insane emperor ordered all religious statues beheaded… and that's something that's mentioned the first time the MC encounters a headless statue in the story, and only then. Otherwise, the detail wouldn't have fit.
(By the way, The Emperor's Edge is free, and I recommend the series for anyone who likes Patricia Briggs' traditional fantasy.)
That brings us to #2:
Your setting must keep the reader's interest.
That means it has to fit the context, like the aforementioned explanation of "Stumps." It also has to be suitably short and interesting.
In other words, don't write an essay or a shopping list—or a tirade about the evils of child slavery. (See the next point for an exception.)
Give your readers some credit; they have imaginations, too. A story is also not a movie. The reader needs to see the details they won't assume, not every single detail. (For more on that, see Janice Hardy's recent post.)
Exceptions to the above points come from #3:
Your setting must fit your story's POV.
Your choice of narrator will color how you must describe things. An omniscient narrator or "distant" POV is more difficult, because the narrator's barely there, so you have to carefully balance and consider what the narrator needs to say and what the author wants to say.
If you write with a "close" POV—meaning everything, even the narrative, is filtered through the POV character's "head" and "voice"—then you can get away with a lot more. You still must be careful to ensure that things fit and that the description stays interesting, but the character's "voice" help it be interesting.
For example, take the character River Tam from Firefly (TV series) and Serenity (movie). I could describe her as a young woman, a savant, who's been surgically altered by the government to be a telepathic fighter, who lacks mental shielding and whose doctor of a brother gave up everything to rescue her and try to keep her safe. That's short, gets the gist, but it also sounds a little like a dossier.
If I were to describe River Tam in one of the Destiny Walker books, Destiny would probably say something along the lines of: "River Tam: neurotic teen with extraordinary reflexes and killing ability. Sounds like me."
Destiny wouldn't really be interested in Simon (River's brother) or in the Hands of Blue (the folks who messed River up). So having her mention one of those two wouldn't "fit" her POV.
Do you tackle the editing for your setting? Do you find it easier to establish setting for the "world" itself (big picture) or for each individual scene (little picture)? Do you prefer writing and reading a "distant" (formal) or "close" (informal) POV?