The 3-Point Lighting Principle

Key Light:

This is the main light. It is usually the strongest and has the most influence on the look of the scene. It is placed to one side of the camera/subject so that this side is well lit and the other side has some shadow.

Fill Light:

This is the secondary light and is placed on the opposite side of the key light. It is used to fill the shadows created by the key. The fill will usually be softer and less bright than the key. 

Back Light:

The back light is placed behind the subject and lights it from the rear. Rather than providing direct lighting like the key and fill lights, its purpose is to provide definition and subtle highlights around the subject's outlines. This helps separate the subject from the background and provide a three-dimensional look and feel.