A website can be viewed on all types and sizes of screen. Responsive websites will protect the readability and usability of the website’s content when it is shown on different devices. The content flows like a fluid, filling the screen, no matter the size or shape.
There was a time when web pages were built to fix sized browser windows. You may still come upon websites that are fixed width and the text looks tiny on your smartphone, or unable to properly fill your desktop browser window.
But now, the world has exploded with devices that can access the Internet with all kinds of screens (or no screens at all). Websites that follow Responsive Design will work no matter what kind of screen displays the web page, whether a large TV Screen or a small phone. The content will flow into the screen space and still retain the carefully laid out design.
You can see it in action on your desktop computer by resizing the browser window that is holding this article. You can change the size of the frame, and see the website flow into any configuration.
If you are reading this on a smartphone or tablet, you can flip the device 90 degrees to cause the screen to fill horizontally and vertically and you will see everything reshape itself.