Well on a mobile-friendly website, the pages and content of the site automatically respond to whatever device they are being displayed on. A number of different things will change on your site including some of the following; the website menu might become a burger menu instead of listed across the top of the screen, images may become smaller, content will be stacked on top of each other instead of displaying side by side. A key feature of a mobile-friendly site is not needing to scroll horizontally. Your website should be able to read the screen width on the device it’s on and respond accordingly.