A responsive page or app is an interface that automatically adapts its layout to the size of the screen displaying it, from smartphone to large monitor, without losing readability or ease of use. This is known as responsive web design.
In practice, the front-end developer uses CSS to rearrange elements based on available width: columns stacking on top of each other, a menu that collapses, resized images, adjusted text sizes. The key tools are flexible layouts (Flexbox, Grid) and media queries.
A large share of web traffic now comes from mobile, and many teams design for the small screen first (mobile first) before scaling up. Responsive is no longer an option but a baseline expectation.
Why it matters when hiring
Responsive design is a prerequisite for any front-end developer, not a resume selling point. Real skill shows on the hard cases: data tables on mobile, complex forms, accessibility, performance on slow networks. Ask the candidate to show an interface they made responsive and the trade-offs they negotiated with the designer.