Flash units are devices used in photography to provide a burst of artificial light, helping to illuminate subjects in low-light conditions or to freeze motion. They can be built into cameras or used as external accessories, allowing photographers to control the intensity and direction of the light. These units come in various types, including speedlights, studio strobes, and ring flashes, each serving different purposes. Flash units can enhance image quality by reducing shadows and improving color accuracy, making them essential tools for both amateur and professional photographers.