Dosage is the amount of medicine you take and how often you take it. The recommended dosage for your antibiotic might be one pill twice a day for a week, for example.
Prescription medications, as well as drugs like aspirin or cough syrup that you buy over the counter, include the dosage information on the bottle or container. Nutritional supplements, such as vitamin C, also include a dosage or a suggested quantity to take. In medicine, dose is used to mean "amount" and dosage to mean "frequency."