This is one that was established in April 21st, 2015 as a ranking factor for determining if the user interface and user experience is impeded on small size screens such as mobile phone and tablets.
From 2018 Google is now a “mobile first” search engine – judging websites by their ability to server mobile users as a priority.
The term “mobile friendly” merely suggests that there is some implementation strategy in the website that makes it possible for mobile users to interact with the content.
There are several ways in which to achieve mobile friendliness but here are the top two:
1. An additional site may be built especially designed for rendering on smaller screens. This involves duplicating or producing new content in an alternative domain, sub-domain or directory that shows users of mobile devices a different URL and set of content to users of desktop computers. The major pitfall here is that it requires two sites to be built and may double the amount of work in building and managing the sites’ content.
2. The better option (in my view) is to build a single website that serves the same content to both large and small screens, but that re-formats the content dynamically to fit the screen size. This is commonly referred to as “responsive design”, because the layout responds to the screen size. With many responsive designs, it’s sometimes still prudent to block some elements in the website from rendering on the smaller screen devices in order to speed up services to small screens, or to eliminate elements from the page that don’t function well when re-configured in size.
The most efficient and effective transition from not mobile friendly to mobile friendly is done via method 2. This is because no rank loss occurs and no 301 redirects are needed for Google-bot mobile to begin assessing mobile rank. All other things being equal, the current rank is preserved and the rank improvement in mobile search can begin from its current position.