“She wrote me a prescription for some pain killers.”

You had a pain in your shoulder. You went to see a doctor, and she gave you some medicine. Now you've returned home and you're telling your roommate what happened. You say:

She wrote me a prescription for some pain killers.

(a doctor) wrote (someone) a prescription for (medicine)

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.

some (something)

You use "some ___" to talk about an amount, or a number, of something, that's not specific. For example:

I met some really nice people in Finland.

This means that you met more than one nice person, but it's not said how many.

You can use "some" with countable things, like people. Or you can use it when talking about something that you can't count. Like when you're making someone a cup of coffee, you ask:

You want some sugar in it?

a pain killer

A "pain killer" is a kind of medicine that's supposed to make your pain stop or decrease.