When a doctor thinks that a patient needs medicine, they will write a note explaining which medicine, and how much to take, which the patient takes to a pharmacy. This note is called "a prescription". The doctor's action is expressed as "writing a prescription". Notice the different way this phrase can be used:
She wrote a prescription.
She wrote me a prescription.
She wrote me a prescription for some antibiotics.
She wrote a prescription for some antibiotics.
You can also use the verb "prescribed":
The doctor prescribed a pain killer.
But "prescribed" sounds a little more formal.