Mobile-friendly is a website or an application designed for use on mobile devices such as smartphones and tablets. The design and layout of the site or app adjusts to fit smaller screens making it easier to navigate and read content.
A mobile-friendly website focuses on providing a good user experience on mobile devices, while a mobile-responsive website goes a step further by adapting its layout to suit specific screen sizes and orientations of a device. In general, a mobile-responsive website is considered a more advanced design for ensuring a better user experience for mobile devices.