This simple addon collapses long areas of content on mobile devices into an expandable block and "Read more..." button. This feature is very similar to the widely seen "Read more" links on Wordpress blogs and many articles on the web.
How else is this addon useful?
Very long posts or articles can actually actually cause what is known as CLS (Content Layout Shift) and LCP (Largest Contentful Paint) issues in page performance tools, such as Lighthouse. These are also factored into your site's performance in Google Search Console.
By collapsing long content into collapsible blocks, your page can render faster in the visitor's browser, and increase your page performance score!