Responsive design is a way of building websites so the layout, images, and navigation automatically adapt to different screen sizes — from phones and tablets to laptops and large monitors.
Instead of creating a separate mobile site, one set of pages flexes with the device. A fluid grid — often built with Flexbox or CSS Grid — plus fluid images and CSS media queries rearrange content so text stays readable and buttons stay tappable. Done well, it improves usability and UX because people can complete tasks without pinch-zooming or horizontal scrolling. Heavy images should use lazy loading. The same rules apply when the page is a web app, not only a brochure. When the layout cannot flex, a website redesign is often the real fix.
A real-life example of responsive design
Think of a restaurant menu printed on stretchy fabric. On a big table it spreads out with room for photos; on a small tray it folds into a compact list. Same menu, different arrangement. A responsive site does that with columns, navigation, and images.
Google favors mobile-friendly sites in search, and visitors abandon pages that feel broken on their phone. For the practical how and why, see Why Your Website Needs Responsive Design and flexible images in responsive web design.