Not a dumb question. Responsive design is a modern web development technique for building sites that adapt to the screen on which they are being viewed. The idea is to provide a good user experience on mobile, desktop, and tablet with only a single web site (as opposed to detecting mobile/tablet devices and redirecting to a mobile-only version).
To see it in action, go to the link in the OP and just resize your browser window down to tablet size, and then down to mobile.
This is accomplished primarily through CSS3 media queries and fluid layouts (using percentages for element widths instead of fixed widths). You can read about CSS3 media queries here:
