This extension is useful to help you keep track of where you left off when scrolling through websites. If you were to use the Back button, you would lose your place when you switch between pages, and it would be annoying to have to find where you left off again when you switch back. The Scroll Marker makes it simple to remember where you left off since it uses a customizable line to display at the bottom of the web page. This allows you to easily see where you are and how far down the page you have to scroll. 

If you want to change the color, opacity, width, or distance of the mark, all you need to do is to drag the slider.