Responsive design helps automatically resize and reconfigure depending on the screen dimensions of the device they are displayed on. By designing using a flexible grid, a single site design can optimize its user interface to display on mobile phones, tablets, or desktop computers equally well.
The key to remember here is that with the rapid increase in the spectrum of screen sizes on devices today creating a different design for a website on each different device is impractical. Responsive design addresses this problem head on.
To learn more about how to build a responsive website, read about Media Queries.