The Multiplier value in every light lets you increase the intensity, or brightness of the light beyond its standard range.
Left: Default light multiplier of 1.0
Right: Multiplier of 5.0 causes burned colors
Since increased Multiplier values tend to wash out, or "burn" portions of the image, you're better off adding lights, or reducing the intensity of other lights when you need to adjust the brightness of areas in your scene. Remember that you can adjust the intensity of a light using its V(alue) spinner. In most cases, it's better to adjust the V spinner than to alter the default Multiplier value.
Left: Spotlight with negative multiplier subtracts light from the scene.
Right: Multiplier of 0 and a negative density on a shadow whose color is white creates the effect of a negative shadow.
An unusual characteristic of the Multiplier is that you can use negative values to create negative light. You can use negative lights to further control the lighting in your scene. For example, you might want a darker area in the corner of a room.
A negative Multiplier value reverses the color of the light, so a red light would become cyan (the complementary color). In addition, the map image in a projector light becomes a negative image.