this bookis intended to teach how you expand sentences by providing more information in senetnces that lack some details.
Descriptive words in writing add details to a scene or action by making the imagery in it more precise for the reader to visualize. For example, sentences with a person waiting patiently or nervously for something to happen probably lead to very different paragraphs or stories. Maybe it's significant in a mystery novel that something happens by a stone wall rather than a clapboard wall.
Descriptors can also add layers of meaning to a scene, or set up metaphors, with just one word. A character with Victorian sensibilities gives the reader a very different feeling than one with punk attitudes.