Responsive web design (RWD) is an approach to web design that makes webpages render well on a variety of devices and window or screen sizes. ... Media queries allow the page to use different CSS style rules based on characteristics of the device the site is being displayed on, most commonly the width of the browser.
What Is Mobile Responsive Design?
When a website is responsive, the layout and/or content responds or adapts based on the size of screen they are presented on.
A responsive website automatically changes to fit the device you’re reading it on.
Typically there have been four general screen sizes that responsive design has been aimed at: the widescreen desktop monitor, the smaller desktop (or laptop), the tablet and the mobile phone.
As you can see in the examples below, as the screen gets smaller, the content shifts and changes to the best display for each screen.