I’ve been searching for single-post and single-post script and nothing is coming up for me. I guess it could possibly be specific to some CMS? The article just isn’t giving me a lot to go on.
I’ve only seen it in wordpress. That instruction sure sounds like the author is talking about a WordPress site. In WordPress, a page is another type of post. If you inspect a WordPress site, there’s all kinds of weird class names like .blog-layout-standard, .su-note-inner and so forth.
Just to add to what has been said, WordPress usually has one or maybe more pages devoted to displaying blog posts, usually showing just an excerpt. If you click on one of the posts a new page will open up displaying the entire blog post. On this page WordPress adds the class “.single-post” to identify the page as such.