The Active D-Lighting system within your Nikon D-SLR allows you to manage the exposure of your images more precisely when faced with challenging lighting conditions. With Active D-Lighting you are able to control high contrast lighting to maintain shadow and highlight details that are often lost.
When you are shooting in strong lighting conditions, the contrast between bright and dark areas of an image can be quite extreme. Active D-Lighting allows you to control this to ensure that your resulting exposure has detail across both the shadow and highlight areas.
If you compare the two images, the one on the left is when Advanced D-Lighting is turned on. The image on the right was taken with D-Lighting turned off.