Description lists the appearance, smell, mood, or other characteristics of something. Descriptive text might account the appearance of a person (he had blue eyes, a big nose, and curly brown hair), or the details of a location (two stands of tall oak trees dripping with ivy lined the long, narrow driveway), or some other element that lets you immerse yourself into the story. Descriptive text adds the details that help you visualize the characters and events in your mind's eye. It — quite literally — describes a person, place, or thing.
What's the difference between description and narration?
Narration is the act of giving an account. The narrator is the person or entity within a story that tells the story to the reader. First-person narration presents the story from the viewpoint of one of the characters ("I maintain that the Ewell's started it all, but Jem . . . said it started long before that" . . . From To Kill a Mockingbird). Third-person narration presents events from the viewpoint of the writer, as if watching the story. ("She slept lightly at first, half awake and drowsily attentive to the things about her." . . . From The Awakening).
Generally, third-person narrative allows the writer to comment on characters and events to the readers, while first-person narrative unfolds as the narrator sees, or comes to understand, the events.