What is the TL;DR of the category? What do tools in this category let you do?
These tools allow you to write CSS in a different syntax to allow for variables, nesting, and other helpful features. The code written with a preprocessor language will be converted to plain CSS so it can be understood by the browser.
Why would I need to use it? Why would I need to start using it?
These tools are helpful for when you want to start writing CSS with variables or functions and have cleaner code.
Where in the CSS design/pipeline is this category of tools used? Learn more.
The preprocessed CSS code must be converted into plain CSS using a compiler. This compiler must convert the CSS before it is sent by the web server.