Previtamin D is an intermediate in the production of cholecalciferol (vitamin D).  
It is formed by the action of UV light, most specifically UVB light of wavelengths between 295 and 300 nm, acting on 7-dehydrocholesterol in the epidermal layers of the skin.
The B ring of the steroid nucleus structure is broken open, making a secosteroid. This then undergoes spontaneous isomerization into cholecalciferol, the prohormone of the active form of vitamin D, calcitriol.
The synthesis of previtamin D is blocked effectively by sunscreens.