Responsive web design makes your web page look good on all devices: desktops, tablets, and phones. Your web page should look good, and easy to use, regardless of the device. Responsive web design uses only HTML and CSS.
Web pages should not leave out information to fit smaller devices, but rather adapt its content to fit any device.
It is called responsive web design when you use CSS and HTML to resize, hide, shrink, enlarge, or move the content to make it look good on any screen.