Responsive web design is an approach to web design that makes web pages render well on a variety of devices and window or screen sizes. It involves using flexible grids and layouts, images, and CSS media queries to adapt the presentation of a website to different device types.

It’s important for modern websites because users access content on various devices, such as smartphones, tablets, and desktops. Responsive design ensures a consistent and user-friendly experience across different screen sizes, improving accessibility and usability. It also contributes to better search engine rankings, as search engines favor mobile-friendly websites.