Heads up! this post was created when Microthemer was at version 2. The current version is 7. Some references to the interface may be out of date.
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.