First and most obvious among the basic functions of stage lighting is visibility. In order to enjoy the show, people in the audience must be able to see the actors on the stage. However, that is only part of the equation; in addition to making sure actors can be seen, lighting must also ensure that the parts of the stage that should not be lit are properly darkened. This concept can be referred to as "selective visibility."
Lighting plays an important part in making the setting of scene as believable as possible. For instance, if a scene is taking place in a room at night, the light must appear to come from a chandelier or some other light source within the room. Basically, at all the times the lighting sources must make sense within the context of the play. This concept is sometimes called "motivation," but can also be called "naturalism" or "illusion of nature."
Composition is the area where lighting and scenic design intersect in the art of theater. The lighting designer must enhance the conceptual ideas of the scenic designer whenever possible. Where the stage director has blocked the actors is also extremely important in this aspect of lighting design; it is the lighting designer's job to place lights in the places where the actors will be. For instance, if the primary action of the scene is taking place in the foreground, the lighting designer's job will be to put light there, where the action will be, and make sure the back areas of the stage are not receiving as much light as the foreground.
The enhancement of mood works in connection with all of the other functions of lighting. Basically, it is the lighting designer's job to express the overall emotion or feeling of a scene. This may involve having a certain color of light, a certain level of intensity or varying movements of light upon the stage.