ND filters, or Neutral Density filters, are optical devices used in photography and videography to reduce the amount of light entering the camera lens. They come in various strengths, measured in stops, allowing photographers to control exposure without altering color balance. This is particularly useful in bright conditions or when using wide apertures for shallow depth of field. By using ND filters, photographers can achieve creative effects, such as smooth motion in flowing water or a dreamy background blur. They also enable longer exposure times, which can enhance the overall aesthetic of an image while maintaining proper exposure levels.