Responsive Web Design (RWD) is a new approach to designing websites efficiently size and stretch themselves to the screen without intervention. With RWD, websites will load in an optimal state on desktop browsers, tablets, and mobile devices. RWD minimizes the need to enlarge or horizontally scroll by dynamically placing content in smartly sized components.
They made this change because they want web users to have a better experience when browsing sites online. With the explosion mobile and tablet browsing, it’s more important than ever to focus on making content available and optimized to a wide variety of popular browsing platforms.