Rim lights illuminate the object from behind or from one side or the other. They are useful to separate objects from the background, making them 'pop' out which can be useful to convey depth and spacing in the scene.
Rim lights are also commonly referred to as Back Lights.
Rim lights can be added to a scene through the Phong MAT material. Each Phong MAT can create a maximum of 2 rim lights, which will be rendered only on that material.
Also see Using Rim Lights for an example of these features.