Instead of just swapping the images this script fades smoothly from one image to the next. For browsers that do not support opacity, the script degrades to a simple image swap. The principle behind fading rollovers is that you have 2 images 'layered' over the top of each other. When you mouse over the image the foreground images fade from completely transparent to completely opaque (solid). In this way the user sees the background (underneath) image first and as the foreground (top) images starts to fade in it appears that the background image fades to become the top image.