"GraphEdit is a visual tool for building and testing filter graphs. It is provided as an executable with the DirectX SDK. With GraphEdit, you can quickly build and test filter graphs to see if they function as you expect. You can even view a filter graph created by an application running in another process." (Taken from the GraphEdit help file)
A filter in this case is the front end of a codec. GraphEdit lets you visually connect codecs together, allowing you to override default priorities of the codecs. Also allowing access to disabled codecs. GraphEdit was designed for the testing of codecs.