"For years, the Internet has been about web sites and browser-based experiences, and the systems that drove those sites generally matched those experiences. But now, the portable world is upon us and it is formidable."
Can I get a quick explanation?
In a nutshell, responsive web design means to create a website that can adapt to different sizes of screens.
Technically, there is a bit more to it than that; it doesn't refer to the size of screen, it alters depending on the resolution of the screen which is currently the best indicator as to what size of device is being used to view the site. It helps to eliminate the need to have a specific mobile site, avoiding having users scroll unnecessarily and generally help users on smaller devices use the website more intuitively.