Vitamin D is the one vitamin your body can technically make on its own, but only with enough direct sunlight on your skin. In the UK, that's a problem for about half the year.
D3, or cholecalciferol, is the form your skin actually produces. It's also the form found in animal sources like oily fish and egg yolks. The other version, D2 (ergocalciferol), comes from plants and fortified foods but is less efficient at raising your blood levels of vitamin D.
Once it's in your system, Vitamin D works as a hormone more than a vitamin. It supports normal immune function, helps your body absorb calcium and phosphorus from food, and contributes to maintaining bones, teeth and muscle function. Low levels are linked to everything from fatigue and low mood through to weakened immunity and muscle weakness.
The NHS officially recommends that all adults in the UK consider taking a Vitamin D supplement from October through to early March, because sunlight at our latitude during those months is too weak to produce enough. Most multivitamins only hit 100% NRV, which is a maintenance dose. We use 200% because if you've been short, you need more than maintenance to catch up.