A few years ago, responsiveness was considered as an added bonus in a WordPress theme. However, with the evolution of various devices, having a responsive website has become the need of the hour.
Responsive Web Design (RWD) essentially indicates that a web site is crafted to use W3C CSS3 media queries with fluid proportion-based grids, to adapt the layout to the viewing environment, and probably also flexible images. As a result, users across a broad range of devices and browsers will have access to a single source of content, laid out so as to be easy to read and navigate with a minimum of resizing, panning and scrolling.
—From Wikipedia, the free encyclopedia—