You use "description" when you give details about how something looks, feels, sounds, tastes, or smells. You're basically painting a picture with words so that the reader or listener can visualise or sense the thing you're talking about. For example, if I were describing a morning in London, I might say, "The streets glistened wet from the night’s rain, and a gentle fog blurred the outlines of the distant buildings."
"Explanation" goes a step further by answering why or how something happens. When you explain something, you are linking facts and information to show causes or reasons. Using the same London scene, an explanation might look like, "The streets are wet because it rained last night, and the fog is typical of London’s weather, often caused by the city’s location near the river and the cool temperatures in the early morning."