The highlighter simply ‘highlights’ which element(s) on the page the current selector targets. Selectors allow you to targeting different things on your page e.g. a post, or a set of posts, a header, footer, sidebar, contact form – you name it. You need to target a page element with a selector before you can restyle it.
You can toggle highlighting ON and OFF by clicking the icon . Turn it on to remind yourself what a selector refers to on the page, then turn it off to see clearly the changes you’re making as it can obstruct the view.