"Responsive Web Design" is a method of designing websites so that they are as user-friendly as possible on all screen sizes, including computers, laptops, tablets and smartphones. This means that very often the layout and design of a website will change depending on the screen size of the device being used (for example, some columns that are visible when viewing a website using a laptop may not be present when viewing it using a smartphone). This removes the need for mobile-users to manually zoom-in to be able to view parts of a website.
Here's an example:
