It probably helps to think of a neutral density filter as a pair of sunglasses for your lens.
A neutral density filter blocks a portion of the light from passing through it (the ‘density’) without affecting the colour (hence ‘neutral’).
ND filters are usually produced in densities that are measured in stops of light reduction. An ND 2 filter allows half the light to pass through it (so 1-stop less light reaches the sensor); an ND 4 filter allows a quarter of the light to pass through (a 2-stop reduction); an ND 8 transmits an eighth of the light reaching it (a 3-stop reduction).
In this way, an ND filter effectively enables you to use a longer shutter speed and/or a wider aperture at any given ISO setting. You can also stack multiple filters to extend exposure times further still, although doing so can degrade image quality.
ND filters can also be used to darken the existing illumination in order to use flash as the main light source. This is an interesting way to record nearby subjects, such as a portrait shot in the bright sun. The ND filter is used to reduce the background exposure to a point that is equal in intensity to (or darker than) a flash exposure that has been calculated to produce a correct exposure with the ND filter in place. The result is a well-lit subject against a balanced—or at least controlled—background. It is also possible to adjust the output of the flash to give a similar rendition without a filter.
Focus in Photography is John Neel’s exploration of advanced focus that will greatly enhance your skill as a photographer. These pages will show you everything, from high-end techniques to create infinite focus, through to using it as an artistic tool, directing your viewer’s eye around your photographic composition.