A null is a custom object, that doesn't render, but is visible in the scene. In other software packages, nulls are commonly used for camera and spotlight "look at" targets, for example. A wfmm null is visible in the scene as a small star object, almost a 3D asterisk.
Some people use a simple primitive, such as a cube, painted completely transparent, as a pseudo-null in their scenes. This does work, but has problems - you have to make sure you disable it from any collision, and it can be inadvertently included in custom scene exporters, etc. While not major problems, they still require a bit of extra effort to work around them, impeding workflow. With cp4's null, you just click.