Very simply, the descriptive approach describes how lanugage is used without saying whether it is "right" or "wrong," while the prescriptive appoach says that something is correct or incorrect.
For example, if you ask me whether you can say "I seen it," my descriptive answer is "It wouldn't be considered standard business English and is almost never found in formal writing, but you will certainly hear it, particularly in rural areas of the U.S." My prescriptive answer would be "For the love of God, no! Don't ever say that!"
[not a teacher]
Student or Learner