Web pages are viewed using many different devices such as Desktop PCs, Tablets, and Mobile Phones.
Your website should look good, and be easy to use, regardless of the device it is being viewed on.
It is called Responsive web design when you use CSS and HTML to resize, hide, shrink, enlarge, or move the content to make it look good on any screen.
Play with your website. Use a Desktop PC, Tablet, and Mobile Phone and see how a single page looks on the different devices.
Pretend you are a customer and go through the processes of looking for products and/or services.
Remember to test the shopping cart process (if you sell online).
Make sure you are not losing customers at the critical payment stage.
Google website tests use the mobile platform by default.
It a priority now to make sure your website looks good on a mobile phone.