postcss-loader processes CSS files with PostCSS. It enables the use of PostCSS plugins within your webpack workflow, allowing you to transform your CSS with tools like autoprefixer, cssnano, and more. Integrating seamlessly with webpack's module system, it accepts various options to configure your PostCSS environment, like custom plugins and parser/stringifier. This facilitates modern CSS development and optimization directly within your build pipeline, supporting sourcemaps and other webpack features.