Responsive web design is when a website acts and looks different on a mobile device than on a laptop or computer. It’s really important, not just for your Google rankings but also for your viewers. For instance, buttons are bigger, text is set out differently so it’s easy to read, and you’ll have a ‘burger menu’ (three horizontal lines) instead of a menu across the top bar. If all the elements of a desktop design stayed the same on a mobile device, the website would be unusable.
Even better than responsive design, is 'optimised for mobile'. While responsive designs adapt the layout to fit the screen, a site designed and optimised for mobile goes a step further and changes font sizes, loads different images, and may turn off certain features like animation or swap videos for static images to help improve load times (and to increase the chance of getting found in a Google search on a mobile device).