When creating a web application, or a web site with more than one page, you will need to reference different resources. If you create a blog, you need to create unique paths to all the blog posts, like we are doing in this Christmas calendar. You see that the url is https://security.christmas/2018/20, where 2018 is a reference to the year, and 20 to the day of December. It is a fairly simple system, and you may have tried to skip ahead, but been met by a page saying you have to wait a bit longer?
A hot topic these days, and I don't think we'll ever agree, but here are some ways to do CSS in your React app