Responsive design is designing one web page to be easily viewable on multiple screen sizes.
Ever tried to visit The New York Times on your iPhone? Since The Times uses the same version of the site on your laptop as it does on your iPhone, you need to pinch and zoom and scroll to get over to the story that you want to read. Then, you have to struggle a little bit more to resize the text so that it’s readable. For The Times and many other web sites, you need to go to their mobile app to get a good reading-on-your-phone experience.
It doesn’t have to be that way!