To prescribe is make orders or give directions for something to be done. These days, the word is mainly used by doctors who prescribe medications to take.
Doctors do a lot of prescribing: they prescribe drugs, rest, exercise, and getting rid of bad habits like smoking. When a doctor prescribes something, he or she is saying, "You need to do this. You should do it." That's the most common use of prescribe, but it pops up anywhere someone is advising or ordering someone to do something. All laws and rules prescribe things — they tell you what to do.