RWD Intro is basically introduction of responsive web design (RWD), a set of practices that allows web pages to alter their layout and appearance to suit different screen widths, resolutions, etc.
RWD Intro- Concept
Responsive web design (RWD) is a web development approach that creates dynamic changes to the appearance of a website, depending on the screen size and orientation of the device being used to view it. RWD is one approach to the problem of designing for the multitude of devices available to customers, ranging from tiny phones to huge desktop monitors.
RWD Creating User Experience
RWD ensuring that elements fit within a page is not enough. For a responsive design to be successful, the design must also be usable at all screen resolutions and sizes. When elements move around the page, the user experience can be completely different from one view of the site to the next.