Responsive Design is a web design approach that prioritises the way a website adapts when viewed across an array of devices. An example of this would be having a website that is comfortably viewable on both a desktop and a smartphone or tablet.
Responsive Design is important as it allows all users to have a similar experience regardless of what device they are using to visit the website. As a user’s experience needs to be consistent, it is important that the site is functional across any device they may need to use the site from. If you have to pinch the screen to zoom, or scroll from left to right, then the website isn’t responsive.