Today, I was sorting through my gear and putting my travel equipment together based around an Olympus Micro 4/3 camera. I put a Lee Filters ND.9 Hard Grad onto the lens and took a couple of pictures off my balcony as a test. A graduated neutral density filter is a piece of glass that varies in the amount of light it lets in across its surface.
Think of sunglasses that only the top of the lens is dark and the bottom of the lens is clear. The boundary between the dark area of the glass can vary from a hard transition to a much softer, or gradual transition. Graduated neutral density filters also come in different strengths as to the amount of light they block. The Lee Filters ND.9 equates to a three stop block of light in the dark part of the filter.
Graduated neutral density filters are useful for landscape shots where there is a large variation in the brightness of the foreground subject and the background, such as the sky. With a normal exposure, exposing for the sky can result in a very dark foreground. The filter allows the shadows in the foreground to be brought up in exposure without blowing out the sky.
Below are two jpeg examples, straight out of the camera, that show the difference between using a filter or not.
Above: Image taken with no filter
Above: Image taken with graduated neutral density filter