HTML stands for Hypertext Markup Language and is the standard format in which documents like websites are rendered and displayed in web browsers.
CSS stands for Cascading Style Sheets. It is used for styling websites. By using CSS-File you have a clear separation of document (HTML) and styling (CSS).
HTML/CSS Themes are the most basic website themes and templates you can get. They are not coupled with a content management system or something else. It is just a bunch of HTML and CSS files which are formatting the layout and design of your website.
It is just a bunch of HTML and CSS files
Actually this makes pure HTML/CSS themes extremely flexible as every theme can be adapted to be used with a content management system like Wordpress or any other system, e.g. also for static site generators.
If you want to develop a very simple website which is not overly complex regarding structure, number of pages and user interactivity, it is perfectly fine to just use a HTML/CSS theme and write your content directly into the HTML files. Of course this requires some knowledge about basic web technologies and is not suitable for everyone.
Get started with web development and HTML and CSS by watching some beginner tutorials on Youtube: