The silver crystals in these areas are activated and turn darker. Other crystals that were not exposed keep their lighter color. Thousands and thousands of exposed and unexposed crystals will eventually form a pattern that can be seen as a photographic image. As long as the film is not re-exposed to light, the image remains on the film. Photographers call this a latent image.
Film development is now necessary to keep the image in place.
All stages in film development are completed in the dark, so that the image is not destroyed. The film is first placed in a chemical bath of developing solution. The developing solution alters all the crystals that were exposed during picture taking and makes them darker. The fixer removes all the unexposed crystals from the emulsion layer so they won't be exposed later. If you think about, in the negative, bright areas were recorded as dark, while dark areas were recorded as light.