It means, in simple terms, that the website is adapted to browsing on mobile devices such as smartphones or tablets. This also means that it was created in accordance with the latest Google standards, i.e. based on RWD (Responsive Web Design) techniques. However, it means much more than just that you can “open” it on your smartphone. A fully responsive website will be “friendly” to this device, i.e. the texts will be clear and legible, and the graphics will be “arranged” to fit the size of the screen. In addition, the whole thing will be able to browse efficiently even on the smallest mobile device.