One page, every screen
People visit your site on 340px-wide phones, 1440px laptops, and 4K displays. 'Responsive design' means one HTML file that adapts to all of them — no separate mobile site. It rests on three ideas: 1. A viewport meta tag so mobile browsers stop lying about screen width. 2. Flexible units (%, rem, max-width) instead of fixed pixels. 3. Media queries that change styles at defined breakpoints.
why it matters
Over half of the world's web traffic is mobile. A layout that only works on your laptop is broken for most of your audience.