GND filters are pieces of glass which are darkened at one end and clear at the other end. In between, there is a transition point from dark to clear which varies depending on the specific type of GND. They even out the exposure of a scene which naturally consists of a bright section (sky) and darker section (foreground). The human eye is capable of visualising a large range of brightness and shadows. This is known as the ‘dynamic range’ of a scene. Unfortunately, camera sensor technology is not yet capable of capturing the dynamic range which the human eye is capable of. Hence, the use of GNDs assists with capturing the dynamic range of a scene using a single exposure instead of blending multiple exposures with different shutter speeds.
Example of the effect a Medium GND has on exposure on the bright area above the horizon