CSS follows the format:
selector {
attribute: value;
}
Every selector in the stylesheet targets children of some HTML element with the class markdown-body, like I said earlier. That’s what “the prefix” means, it is a class selector.
(If that doesn’t make any sense to you, I suggest you spend 15 minutes reading about the basics of HTML.)
Also, @chrillek wrote a great beginner’s guide to CSS here: