Height fields give you a simple way to create a 3D object with your paint program. However judging from the number of height field questions on the Internet, I would say that this POV-Ray command is very misunderstood. Hopefully this article will clear up some of that misunderstanding. This may sound complicated, but a simple example will show you what this means. Let's say we want to make this greyscale image into a 3D object.
Height fields make the color white the highest point and the color black the lowest point.
In the previous image, I want the light color to be the lowest and the dark color to be the highest points. To accomplish this I have to invert the image. Inverting an image can be done with many paint programs. In this example I used Corel Paint to invert the image. They simply declare the camera and the lighting for the scene.