Stylus is a preprocessor that generates CSS (Cascading Style Sheets). Stylus stylesheets (.styl files) contain code written in the Stylus syntax, which offers features like variables, mixins, functions, and operators to make CSS authoring more efficient and maintainable. These files are not directly interpreted by web browsers. Instead, they must be compiled into standard CSS files before being used in a website or web application. The compilation process transforms the Stylus code into equivalent CSS code that browsers can understand. Stylus aims to reduce the verbosity and repetition often found in traditional CSS, allowing developers to write cleaner, more organized, and reusable stylesheets. It supports both indentation-based syntax and a more traditional CSS-like syntax, providing flexibility for different coding styles. Stylus files are commonly used in web development projects to manage the styling of web pages and user interfaces.