You've doubtless heard of this term before but I don't think I've ever fully realized until now what it means. It's description as clue. Each line fulfills multiple functions. It is even better if each line is precise. Rather than a few vague, sweeping descriptive sentences ("The officer sat fearfully behind the desk in the enormous waiting area, watching over both the large, double doors and the sweeping staircase") it builds in a few precise details that can suggest the rest ("The officer sat stock still and sweating behind the steel folding table in a corner of the enormous lobby, watching over the large mahogany doors and the sweeping marble staircase"). The reasons why the second line is better are multiple:
* Precision in that second line aids author credibility. Authors that speak with precision suggest that they've put a lot of thought into the description. It wasn't just the first thing that flew off the keyboard. So, don't say desk when you mean table. Don't say waiting area when you mean lobby.
* The Description gives a clue to the character's personality or mood. My example line of description suggests fear though the personality clue isn't as strong here as it could be. Perhaps not the POV's fear, but a sense of fear surrounding him/her. The minor character is quite stressed and awkward, the description makes him sound almost caged in.
* The Description assists the reader to make assumptions. The officer seems anxious, which suggests that he fears trouble. Whether the trouble is a superior officer or an oncoming war isn't mentioned. The clash of the table compared to the building gives a clue that perhaps the mansion wasn't always housing officers. Perhaps it was commandeered to the purpose..
* The Description allows the reader to make assumptions. I don't need to belabor the point on how this place is quite rich. I don't need to describe the oil paintings on the wall or the heavy velvet curtains. The readers can assume based on the staircase and the large doors. If there weren't any paintings on the walls, but there was dusty or faded squares of paint where paintings once hung, then that would be a valid and useful telling detail.
* Foreshadows the future. The location isn't safe. People aren't comfortable. It suggests oncoming trouble.
* A sense of history. There is a clash here between what was (a mansion) and what is now (the entrance hall hastily turned into a lobby) and this is shown also through the clashing descriptive elements. Notice how the clue is there and ready to be built upon. I didn't need to begin the scene with information on how 'Ever since the mansion was commandeered three years ago by the harsh commander so-and-so, the officers have been nervously tip toeing around'. I could simply show, bit by bit, through each descriptive element, dialogue, etc. the same thing.
So remember, when writing your description to check if it is precise, provides a clue (foreshadowing, personality, mood), and has a sense of history. Each line doesn't need to do all of these things but the more that the lines, or paragraphs, of description achieve, the better things are.