Preprocessor scripting languages such as SASS/SCSS or PostCSS convert your code into a regular CSS that is recognizable by web browsers. And the process of converting the special type of CSS into regular CSS is known as compile CSS. You can think/read of the word "Compile" as "Convert or Produce or something along those lines." These preprocessor scripting languages are also very similar to CSS, just their pattern or writing style is different. It looks very hard by its name, actually, it's not. See more about SCSS & CSS and how to use them.