A dose is the amount of medicine you're supposed to take. A bottle of aspirin has the recommended dose printed on its side.
When your doctor prescribes medication, she tells you what dose you should take, whether it's one pill twice a day or two teaspoons just before bedtime. You can also get a dose of something else: having x-rays gives you a small dose of radiation, for example, and watching the news instead of your favorite cartoon can be said to give you a dose of reality. The Greek root of dose is dosis, "a portion prescribed."