Film sensitivity refers to the ability of photographic film to react to light. It is measured using the ISO (International Standards Organization) scale, where a higher ISO number indicates greater sensitivity. This means that films with higher ISO ratings can capture images in lower light conditions, while lower ISO films are better suited for bright environments. Different types of film, such as black and white film or color negative film, have varying sensitivities. Photographers choose film sensitivity based on their shooting conditions and desired outcomes, balancing factors like graininess and exposure time to achieve the best results in their images.