Descriptive writing is focused on creating imagery for the reader. This is done through ample details and creative expressions of facts. Think of how events and people are described in news articles. Consider a book you recently read and the descriptions provided by the author to transport you to a new place or to surround you with new ideas. This is one of the great powers of good writing.
Outside of the language classroom, it is unlikely that you will be asked to write a purely descriptive essay during your college experience. Most of the time, descriptive writing is part of a larger writing task.
That being said, control over tense and aspect (narration) are essential parts of descriptions and this writing skill will be necessary throughout your English writing experience. Additionally, the ability to provide details through careful word choice and interesting clausal structures will add to your writing voice and the audience's connection to your purpose as a writer.