It is important to know how to check if your website is mobile-friendly because Google has started to penalize in search results rankings all the websites that do not meet the requirements for an adaptive and responsive design (AWD and RWD).
The adaptive design is set to match predefined screen sizes, while the responsive design is the design that has been created to easily adapt the content and layout to any screen size, in a way in which the website does not lose usability. Both are mobile-friendly.
A mobile-friendly website can be easily visited and browsed in mobile and tablet screens without the need to zoom in the screen. By contrast, a design that is not mobile-friendly will have small fonts and buttons that will be difficult to read and click.
This Google change has made adaptive and responsive design important factors for SEO (Search Engine Optimization). In case we do not take them under consideration, we can lose ground against our competitors.
We can check if our website is mobile-friendly by accessing it through a mobile device, but that could not be enough, we have to make sure that the design adapts to different screen sizes, devices and browsers. It is better to use a tool that Google has especially developed for this task: the Mobile-Friendly Test. We just have to put our website URL, Google will check and will tell us whether the website is mobile-friendly or not.