When you overlay many users' mouse tracks on the web page image, it is easier to see how effective the layout and navigation are. Colors indicate time elapsed from loading the page. The warmest (red, orange) show about first 15 seconds. Then yellow (25 sec.) green and blue (for all above 60 seconds).
Thickness of the track depends on how fast the movement was. We assume that slow motion means greater focus and attention to detail so the line is thick. Clicks appear as small circles.