Vitamin D cholecalciferol-D3, alfacalcidol) is a fat-soluble vitamin that helps your body absorb calcium  and phosphorus. Having the right amount of vitamin D, calcium and phosphorus is important for building and keeping strong bones. Vitamin D is used to treat and prevent bone disorders (such as rickets). Vitamin D is made by the body when skin is exposed to sunlight. sunscreen , protective clothing, limited exposure to sunlight, dark skin and age may prevent getting enough vitamin D from the sun.
Vitamin D with calcium is used to treat or prevent bone loss (osteoporosis ). Vitamin D is also used with other medications to treat low levels of calcium. It may be used in kidney disease  to keep calcium levels normal and allow normal bone growth. Vitamin D drops (or other supplements ) are given to breast-fed infants because breast milk usually has low levels of vitamin D.
Vitamin D is sometimes called the “sunshine vitamin” because it’s produced in your skin in response to sunlight. It’s a fat soluble in a family of compounds that includes vitamins D-1, D-2, and D-3.
Your body produces vitamin D naturally when it’s directly exposed to sunlight. You can also get it through certain foods and supplements to ensure adequate levels of the vitamin in your blood.