Responsive design ensures websites are mobile-friendly by adapting their layout and content dynamically based on the device's screen size and resolution. This approach employs flexible grids, images, and CSS media queries to create a seamless user experience across multiple devices, including desktops, tablets, and smartphones. As mobile internet usage continues to rise, responsive design is vital for enhancing accessibility and user engagement. It simplifies maintenance since a single version of the site can cater to all devices, reducing development costs and improving SEO performance. Ultimately, responsive design is essential for modern web development.