The concept of using a graduated neutral density filter (i.e a grad filter) seems pretty straight forward or at least why you would use one. Say you have a beautiful landscape scene before you however the sky is quite bright relative to the foreground. If you expose for the sky, the foreground will be dark. If you expose for the foreground, the sky will be blown out. A grad filter allows you to darken the sky thereby allowing you to get the whole scene exposed well.
The problem, however, is that you don't just slap the filter on to the front of the camera and expect to get good results. Here is an article on how to properly use a grad filter (and why with more information than my tidbit contains).