The film processing lab develops the filmed footage. This process must be done in complete darkness, as any light would overexpose and destroy the filmed images. The film strip is "fixed" by a chemical process, so it cannot be further developed. Then, the film is copied to a new film strip, because before this stage, the individual images of the film are still negative, i.e. everything is light on the individual frames that should be dark, and vice versa. But by exposing this negative on blank copy, the images are positive again and the images are correct.