The Cokin P154 is a Solid Neutral Density filter providing 3-stop exposure reduction. This creates an overall darkening of the image, allowing you to photograph with a wider aperture or slower shutter speed than current lighting conditions allow.
Slowing your exposure time lets you convey movement in subjects like water and clouds. Increasing your aperture lets you create a shallower depth-of-field which isolates your subject from the background. Neutral density filters do not affect color and can be combined with other filters to create more dramatic effects.