You first know the correct names and terms that catch the emotion or the image. Good description goes beyond accuracy and precision to include the musical qualities of language. The sounds of your words and the cadences of your sentences reinforce the content of your description.
Think of good description as the use of the senses. Your readers need to see things. Here's descriptive writing that makes me feel I'm right in the middle of the dust bowl in 1934 Oklahoma:
Dust coated the dials on the radio, the plates on the table, and the dishes in the cupboards. Evelyn rinsed the lenses of his spectacles, and a few minutes later, she had to do it again.
Are you there? Notice the use of spectacles—which was the common word in those days. That single detail lends authenticity to those two sentences and pulls us into that kitchen.
Good description employs specific, concrete detail for readers to visualize or experience the scene through their senses.