What does the bright white stuff stand for on a xray?

Density: Calcium and barium are probably the two most commonly imaged. They, and other metals are radiopaque, so the xrays are blocked/ diffused and do not get to the film to darken it.

Shadow: An x-ray is the negative image of a shadow cast by a tiny source of x-ray light through the subject. The bright part is the densest material, such as metal or bone, and the dark part has no shadow but was exposed to the full beam. The film is kept in a light-protected environment so that only the x-rays are permitted to expose the film. Most x-rays now are done digitally but concept is same.