Prescriptivism and descriptivism are contrasting approaches to grammar and usage, particularly to how they are taught. Both are concerned with the state of a language — descriptivism with how it’s used, prescriptivism with how it should be used. Descriptivists describe, systematically recording and analysing the endlessly changing ways people speak and write. Descriptive advice is, as Jesse Sheidlower put it, almost an oxymoron. Prescriptivists prescribe and sometimes proscribe, emphasising rules and guidelines based on the conservation of customs (and sometimes a mythical ideal of correctness), and on judging what is or isn’t acceptable — which poses, among other questions: acceptable to whom, when, and why?
Maybe a “descriptivist approach to creative writing” sounds a little oxymoronic, but it gets at how I’ve tried to teach fiction courses in the past, and I think it describes how we learn to write outside a classroom. You read widely, internalize the many ways others are writing, and then synthesize what you read as you find your own voice (or voices). When you sit down to write, you do it contextually, mindful of your own aims and audiences. Supposedly universal doctrines (avoid adverbs, avoid second person, tend toward the minimalistic) are sometimes useful, but usually just the loose consensus of a particular discourse community.
That doesn’t mean you ignore rules altogether. But it means advice like “You have to learn the rules to break them” is maybe a little wrongheaded. A better take might be, “Knowing local customs is useful for a traveler.”