What is prescriptive discourse?

Prescriptive discourse is any discourse that promotes what should be thought, spoken, or done. For example, the sentence 'That window is open' normally expresses a certain descriptive thought (proposition, statement), namely, that that window is open. By way of contrast, the sentence "You should open that window" would normally be used to tell someone what to do, to prescribe a certain action. So prescriptive discourse is normative discourse. It is discourse about what ought to be the case rather than descriptive discourse about what is the case.