Responsive design works hand-in-hand with mobile first as it implies automatic adjusting and re-jigging the content of a website to any device to be a desktop, laptop, smartphone. It can be achieved through using different technologies: e.g. CSS media queries and flexible grids/layouts. As a result you get a great website design each element of which is fully responsive. The main advantage of responsive design is that it allows businesses to pay for just a single site build with the content that displays flawlessly on small- and big-screened desktops.