A Latent image is an invisible image formed on a film negative after it is exposed to light. Unlike digital sensors, film (whether it is for Photography or Cinematography) is made out layers of plastic coated with a gelatin emulsion containing tiny silver halide crystals. These crystals are sensitive to light. When light hits these crystals, a chemical reaction is initiated, which turns their surface into metallic silver. These crystals will later be developed and represent the darker parts on the negative (which are actually the brighter parts in the final image).
The latent Image is the image formed on the negative by the undeveloped silver halide crystals. Therefore, it cannot be seen by the naked eye. It is born right after the camera shutter is released, and stays dormant until the film negative is developed.