Responsive web design allows websites to automatically adjust their layout and content depending on the screen size of the device used to access the site. It uses fluid grid systems and flexible images so that content fits any screen, along with media queries that apply different CSS rules based on screen dimensions. Responsive design is important because most digital interactions now occur on multiple screens and users expect a good experience regardless of device. While responsive design is a first step, adaptive design that changes based on other factors like network speed will be needed to fully meet unpredictable user needs in the future.