Because the Safari toolbar is translucent, colored blocks (or, worse, animated colored blocks) that should be offscreen may become visible through the toolbar when you scroll. This can be distracting for users with accessibility needs. Frosting is an extension that fixes this issue. Some webpages exhibit the problem more severely. This is an exaggerated example.
Try visiting this page with Frosting enabled and disabled. You should notice that the colored blocks no longer bleed through when Frosting is on.