One common issue I encounter when creating landscape, seascape, and nature photography is that the foreground and background light levels are several f-stops apart. This makes it hard for the camera to record what our eyes see. The reason is the digital chips in our cameras have limitations and cannot record the entire range of highlights and shadows.
Neutral Density filters are used to control the light coming through the camera to the chip. There are numerous brands and types of filters but the idea is that the diffusion lowers the amount of light entering so that the chip can record it.
Sometimes it is appropriate to diffuse the entire scene and others just part of it. A graduated neutral density filter only affects a portion of the image. For example when photographing a sunset at the beach you might find that the sky is 2 or 3 F-stops brighter than the beach. Using an equivalent strength filter with the diffusion in the right area will balance the exposure making it easier to record.
A hard graduated filter is best used in situations where the horizon line is definite like at the ocean. A soft graduated neutral density filter is better for areas where the horizon line is less obvious, like when making photographs in the forest. The diffusion in the filter balances and allows the range of highlights and shadows to be recorded.
The image created in Iao Valley stream used a 2 stop graduated hard neutral density filter. The stream created an obvious horizon line.
In the bamboo forest a one stop soft graduated neutral density filter was used to manage the additional light coming through the tops of the trees.