Mobile devices have a smaller screen, are generally touch oriented, and have different user experience expectations than a desktop computer.
To accommodate the smaller screen, we use responsive web design techniques. This means designing the application in such a way that it accommodates the screen size and other device attributes. The idea is crafting websites to provide an optimal viewing and interaction experience across a wide range of devices. The techniques include changing font sizes, rearranging elements on the screen, using collapsible elements that open when touched, and resizing images or videos to fit the available space. This is called responsive because the application responds to device characteristics by making these changes.