Responsive web design is a term that refers to websites that respond to the device a visitor uses to access it. In other words, because of the differences in parsing website data and images between smartphones, tablets, and computers, different website components are necessary to render the sites visible to each device. Responsive design makes your website more accessible to each type of device your visitors are likely to access it from.
The alternative is to maintain multiple websites – one for each type of device. This can be costly, lead to duplicate content issues, and be bad for SEO.
For instance, if you have a mobile website and a traditional HTML website, both with the same content, then Google may decide to de-index one of those sites based on the duplicate content. You certainly don’t want that to happen. If it does, you might as well not have the second website.
Another issue is link building. If you maintain two websites, then you’ll have to build links for both websites. That could be timely and costly. There are much better ways to allocate your resources.
This is where responsive website design comes in. You have one website. That website is accessible from any device. If newer technologies emerge in the future, then those technologies will be rolled into the responsive design model so that your website will be accessible to those technologies. Your website going forward should adhere to current responsive website design standards.