RWD - Responsive Web Design
The technique of designing web pages so that its appearance and layout will automatically adjust
to the window size of the device where it is displayed (eg browser, smartphone or tablet). Page created in this technique is universal and displays well both on large screens (such as Full HD) and on the screens of smartphones or tablets.
A good example of a responsive website is a page that you are currently watching. Catch the browser side edge (left or right), and change the size of the visible window. Page will automatically adjust to the width of your browser . This allows adjust the content in browsers on mobile devices to your browser. And your content will always be legible and will look good regardless of whether you watch it, for example, on your smartphone or on your tablet vertically or horizontally.