Dither is the amount bits used in an image. In most cases you want to use as little dithering as possible, but some browsers cannot support a 24 bit color image. The bit depth is held to 8 bits. The image becomes dithered and you lose some of the image quality and color. Complete this lesson on page 100 (finally at the century mark!) and proceed to the next lesson.
Remember to leave the image and the save for web dialog box open when you move to the next lesson!