A CSS preprocessor is a program that lets you generate CSS from the preprocessor's own unique syntax. There are many CSS preprocessors to choose from, however most CSS preprocessors will add some features that don't exist in pure CSS, such as variables, mixins, nesting selectors, and so on. These features make the CSS structure more readable and easier to maintain.
To use a CSS preprocessor, you must install a CSS compiler on your web server.
Here are a few of the most popular CSS preprocessors: