Your bones, muscles, and nerve fibers all need it. It also helps keep your immune system in its prime.
Your body can make vitamin D when you get enough sunshine. But too much sun is bad for you, so you can also eat foods including salmon, mackerel, and mushrooms exposed to ultraviolet light. Liver, cheese, and egg yolks have small amounts. Milk, some brands of orange juice, and many cereals are fortified with it, too.