PostCSS is a tool for transforming CSS with JavaScript plugins. It allows developers to use modern CSS features and optimize their stylesheets by adding functionalities like autoprefixing, linting, and minification. This flexibility makes it a popular choice for enhancing the CSS workflow. With PostCSS, developers can write CSS in a more efficient way, using plugins to automate repetitive tasks. It supports various syntax extensions, enabling the use of features like variables and nesting, which are not available in standard CSS. This helps streamline the development process and improve code maintainability.