Responsive web design automatically adjusts the layout of a website to display optimally on different devices and screen sizes. It uses flexible grids and layouts that expand and contract depending on screen size, primarily to provide an enhanced user experience for mobile users. The document discusses the importance of responsive design for usability and increasing mobile traffic, how content is optimized and adapted for different screens through layout changes and prioritization of key information. It promotes responsive design training to learn how to build websites that are accessible and easy to use on any device.