Almost every discipline uses abbreviations to keep documentation short and precise. For instance, if you're following a recipe in the kitchen, you'll see "tsp" for teaspoons and "C" for cup. Medicine is no exception. Doctors and pharmacists use abbreviations to describe the dosage of drugs, vitamins and supplements.
Milligrams per day is abbreviated Mg/D. If you're having headaches, your physician may advise you to take 600 Mg/D of Motrin. Or, if you're female, the doctor may suggest that you take 1000 Mg/D of calcium to prevent osteoporosis. The Institute of Medicine also uses Mg/D to quantify the recommended daily allowance for various nutrients. For instance, the IOM notes that adult men need at least 8 Mg/D of iron, while adult women need 18 Mg/D of the same nutrient.
The abbreviation Mg/D does not give you information about dosing frequency. Your physician may want you to take 600 Mg/D of Motrin in three doses -- not all at once. Ask your physician for detailed instructions with any prescription. Do not take supplements without your physician's recommendation.