How To Use an ND filter Pt. 2
An ND, or Neutral Density Filter, is like a very dark pair of sunglasses for the lens of your camera and it's used to reduce the amount of light entering the lens so you can set slower shutter speeds.
They're called 'Neutral Density' filters because they have a neutral colour - ie no colour at all. But they do have density or darkness.
Neutral density filters come in different densities or darkness-es - the darker one is the more light it will stop. My favourite is the Lee Big Stopper which loses ten stops of light and is incredibly dark.
There are many makes of neutral density filters, you don't have to have a Big Stopper to shoot any of the photos in the video. Cokin, Kood and others make them - you could even modify the glass from a welders mask to fit over your lens and use that.
So why would you want to set a slow shutter speed? Shutter speed has only one creative use and that's to freeze or blur movement. If you want to create movement in an area of an image like moving water or clouds then you have to have a slow shutter speed to give the movement time to blur.
I made a film about the Big Stopper with Tom Mackie a few years ago in How to use an ND filter 2 I'm going in deeper to explain things like what makes an interesting ND images how to compose the shot and what questions to ask yourself to find the camera settings you'll need, because you have to shoot in manual mode when using a very dark ND filter.