Vitamin D3 is produced when the body is exposed to sunlight. D3 is also found in some foods including eggs and fatty fish, such as salmon, tuna, and mackerel. D2 mainly comes from eating fortified foods including some types of cereal and dairy.
The most common source of Vitamin D deficiency is not getting enough sun exposure. While malnutrition can contribute to a deficiency as well, the majority of people do not get sufficient vitamin D from their diet anyway. Magnesium is required for Vitamin D to work, so get that tested too.
Vitamin D is necessary for the proper growth and health of teeth and bones. It also helps in the healthy development of the immune system as well as various tissues throughout the body.
If you are low in Vitamin D and have sufficient Magnesium levels, here is a D3 + K2 supplement that I use and recommend.