When you first create a light, its illumination is unrestricted. That is, depending on the light’s type and parameters, it can potentially illuminate every surface in your scene.
You may, however, want to restrict a light’s illumination by linking it to a particular surface (or group of surfaces). If you link a light to a surface, that surface is illuminated only by that light; all other lights are ignored. If the link is non-exclusive, the light also illuminates other surfaces. If the link is exclusive, the light only illuminates the surface it is linked to.
You can link forces to objects using the same method for linking lights to surfaces.