Mobile friendliness, simply put, is the measure of how well a website is designed and optimized to load on a mobile device such as a smartphone or tablet. It layman’s terms, it’s when your site is able to shrink down to fit on any screen while still allowing users to consume content, navigate the site and achieve their goals as easily as with the desktop version. It also means not throwing up intrusive or aggressive interstitial ads that block your content on mobile devices.
Mobile friendliness can be achieved in two ways (more on both of them later):
Using mobile first or responsive web design.
Creating a separate website (usually hosted on a mobile subdomain) optimized for mobile devices and redirect mobile users to that site.
Mobile friendliness does matter for SEO. In fact, it’s a ranking signal in both Google and Bing’s mobile search algorithms and websites that are mobile friendly receive a slight boost to their ranking. So, if your website relies on or targets traffic from mobile users at all, mobile friendliness is vital for SEO.
Even if your business doesn’t rely on or care about traffic from mobile users for some reason, thanks to Google starting its mobile-first index, mobile friendliness will soon matter for you as well.