A responsive theme (as the one used for this website), is an approach to web development that allows a website to break itself down smoothly across multiple monitor sizes, screen resolutions, and platforms, be it a computer, tablet or mobile device. It allows the developer to create a site that is optimized for each platform, both in navigation, readability and load time.

As you can see when resizing the window (if you are viewing this site on a computer), the layout of the page shifts depending on the size of the screen; with different layouts for content depending on viewing area. Themes (or layouts) such as this, allow for a single site and single look to the site, to be viewed on various devices without the need for additional themes or resizing by the user.