More and more projects today are using CSS preprocessors, with Sass being one of the most popular out there. What exactly is a CSS preprocessor? It is a scripting language based on CSS that lets you do amazing things with your CSS. We have a new series out, Learning Sass and Compass, to get you up to speed on this new cool tool for front-end development. "What is a CSS Preprocessor" is a free lesson to help explain what these great tools are and why you want to know how to use them.