If you don’t already know then take a look at your analytics. Â Industry wide the number of people viewing websites from their phones is rapidly increasing and unless you are catering for them then they are probably getting a pretty rocky reception when they arrive.
Modern browsers all support the ability to detect screen sizes and capabilities (such as touch) and then modify the behavior of the website based on this size. So instead of designing a rigid website designed to be optimally viewed at a standard resolution we now design to be flexible and smoothly and gracefully change their dimensions, layout and even content to fit appropriately on whatever device is used.
Read some more on Wikipedia, or get in touch to see how I can help your website work on more devices.