What is a Neutral Density Filter?
A neutral density filter limits the amount of light that passes through your lens and into the sensor. Allowing photographers to control the exposure of their image. Essentially sunglasses for your lenses.
For video, ND filters are extremely useful especially when there is a lot of light outside. Shutter speed needs to be maintained which is usually double your frame rate. For example, if you are going for a cinematic look a popular settings is 24FPS therefore, your shutter speed should be set to 1/50.
What is a graduated ND filter?
It's the same as a ND filter, but graduated! This is basically the ND effect which fades out. This type of filter usually comes in a square format with a holder.
This is widely used amongst landscape photographers who want to capture the foreground as is but darken the sky. This is done by matching the point at which the graduated filter fades out to the horizon of the landscape. Although a disadvantage to this time of filter is it doesn't always line up since landscapes are not always flat.