As more people use their mobile devices to surf the web, websites need to be responsive and display content in the correct size of the device's screen. When a website is designed to be responsive, it adapts the layout to the viewing environment of the user. Responsive websites are deigned by using HTML and CSS to resize, enlarge, move, shrink the content to make it displayed correctly on any screen - mobile friendly.
In short, the layout of the website changes based on the size and capabilities of the device it is being viewed on. Responsive Web Design makes your web page look good on all devices (desktops, tablets, and phones).