For many websites, the majority of visitors arriving at the site - navigating, browsing, searching, trying to find relevant information about your company or perhaps find a product and make a purchase - are using a mobile device.
Mobile devices, such as iPhones, iPads or their Android equivalents, have very different screen sizes to laptops and desktops and also use touch screens instead of a mouse.
These differences may seem obvious but it means that every website needs to cater for a wide variety of different screen sizes or viewports, and for visitors tapping and swiping on their screen instead of clicking and scrolling. And this needs to not only work correctly for mobile users and desktop users but to work well, so there is no confusion or difficulty just because they are using a mobile device.
The best way to make sure your website works well on iOS and Android devices, for smartphone and tablet sizes, covering different mobile browsers, is to test thoroughly.