If you are going to build a new website and which to attract visitors you cannot ignore the one fact, visitors will be using tablets and smartphones as well as a computers. Today the web and mobile smart phones is how they will visit the site and if this is not displayed correctly they most likely will not return.
Responsive design is a concept where you build your website once and then format it so it can adapt to any screen size that accesses it. Designers use HTML5 and CSS to build the sites and set parameters so the content will resize itself whether the user is in vertical or horizontal viewing mode, on a tablet, desktop or smartphone or even a screen as large as a television.
You can tell if a website is responsive by shrinking in your browser window. The content should resize and group into blocks.