Neutral Density Filter
In photography and optics, a neutral density filter or ND filter is a filter that reduces and/or modifies intensity of allwavelengths or colors of light equally, giving no changes in hue of color rendition. It can be a colorless (clear) or grey filter. The purpose of a standard photographic neutral density filter is to allow the photographer greater flexibility to change theaperture, exposure time and/or motion blur of subject in different situations and atmospheric conditions.
The use of an ND filter allows the photographer to utilize a larger aperture that is at or below the diffraction limit, which varies depending on the size of the sensory medium (film or digital) and for many cameras, is between f/8 and f/11, with smaller sensory medium sizes needing larger sized apertures, and larger ones able to use smaller apertures. ND filters can also be used to reduce the depth of field of an image (by allowing the use of a larger aperture) where otherwise not possible due to a maximum shutter speed limit.
Source : Wikipedia
Getting lovely smooth water isn’t simply a matter of choosing a slow shutter speed. No, you’ll need to reduce the amount of light hitting the sensor by using a filter.
The filter in question is a Hoya ND400 9 stop neutral density.