Responsive design is an approach to web design that makes your web content adapt to the different screen and window sizes of a variety of devices.
For example, your content might be separated into different columns on desktop screens, because they are wide enough to accommodate that design.
If you separate your content into multiple columns on a mobile device, it will be hard for users to read and interact with.
Responsive design makes it possible to deliver multiple, separate layouts of your content and design to different devices depending on screen size.