Responsive design means building a website so that it automatically rearranges itself to fit any screen — a large computer monitor, a tablet or a small phone — so it always looks right and stays easy to use.
Think of water poured into different glasses: it takes the shape of whatever container it is in, without spilling. A responsive site does the same — the same content flows neatly into a wide layout on a computer and a tall, single-column layout on a phone, without anything being cut off or too small to tap.
This matters because most people now open websites on their phones. A site that is not responsive forces visitors to pinch and zoom, and many simply leave — so responsive design is considered basic for any modern website.