SASS (Syntactically Awesome Stylesheets) is a CSS preprocessor that extends CSS with advanced features like variables, nested rules, and mixins. It offers two syntax options: the indented SASS syntax and SCSS, which is closer to traditional CSS. SASS helps developers write modular and reusable code, improving project organization and maintainability. By compiling into standard CSS, it ensures browser compatibility. SASS is a popular choice for web developers looking to speed up and optimize their styling process.