Chrome: If you're reading a long article on a computer screen, it can be easy to lose your place while scrolling. Scroll Marker draws a small line at the bottom of your screen to keep your eyes in the right place while you move down the page.
This is basically the digital equivalent of using a bookmark or your finger to keep track of your position on a printed page. When you enable the extension, it will draw a thin line across the bottom of your browser. When you scroll, it will move with the content and remain in place for a few moments to keep you from losing your spot. For such a simple app, Scroll Marker comes with lots of customisation options. You can change the size and colour of the line, how far it's offset from the bottom of the browser window, and how long it stays in place after you scroll.
This is something you'll probably keep disabled most of the time, but it's helpful if you're on a page without much formatting that doesn't give you a lot of visual anchors, or if you're the kind of person that has to keep a finger on the page.