The term ‘Responsive Web Design’ is given to the design approach and philosophy which aims to render websites in their optimal form across different devices. In practice, this often means that elements and content must be moved, adjusted and coded into a page differently depending on whether a web user views that page on a desktop, a smartphone, or a tablet.
Optimising a website for these different devices is important for user experience and to ensure that visits to a given webpage stand the highest chance of conversion. Websites that are not built with this design philosophy will look unprofessional to web users. They are also normally awkward enough to use that users leave the website.