Vitamin D is not one chemical, but many. The natural type is produced in the skin from a universally present form of cholesterol, 7-dehydrocholesterol. 

Sunlight is the key: Its ultraviolet B (UVB) energy converts the precursor to vitamin D3. 

We need vitamin D to help the body absorb calcium and phosphate from our diet. These minerals are important for healthy bones, teeth and muscles.
A lack of vitamin D – known as vitamin D deficiency – can cause bones to become soft and weak, which can lead to bone deformities.
In children, for example, a lack of vitamin D can lead to rickets. In adults, it can lead to osteomalacia, which causes bone pain and tenderness.
Dr. Mohammed Sadiq.