Sites built on responsive framework to allow it to look great on desktop, tablet or mobile devices. It’s like getting 3 websites in one.
Just as an example, the site you’re on right now is responsive. To see it in action, re-size your browser window width down from desktop to tablet, to mobile, and so on.
You’ll notice the site automatically uses CSS3 media queries to determine the width of the browser window and re-orients the page layout so it still looks great even when really, really narrow. The video above also goes a little more into what responsive web design is about and how it works.