A Ghost Topic is simply one you do not want to appear in the Contents. It will be compiled as normal and you can link to it as normal. The only difference between it and a normal topic is that it will not be seen on the Contents list.
Creating a Ghost Topic
Add your Topic as normal, then click on the "Properties" tab below the editor. Check the option "Ghost Topic" and you will see the icon of a Ghost appear next to the Title in the Contents. Un-check this option to make it visible again.