Your website is ranked on Google according to a number of different factors: Keywords in your content, activity on your blog, links and trackbacks to other sites on the web, social media activity etc. A website that has all of these things will rank closer to the top when a customer searches for your products or services.
A mobile-friendly (responsive) website is optimized so that it looks good on all devices. Have you ever gone crazy scrolling back and forth trying to read the tiny text of a website on your phone screen? A responsive website doesn’t have that problem, because it responds automatically to the size of the screen and reorganizes the elements on the page so that it is easy and straightforward to use on any device.
Why does it matter to me?
When a customer searches for you or your products and services on Google, the closer to the top of the list you are, the more likely it is that potential customers click through to your website. If you have paid for SEO or Social Media Marketing and the additional exposure they bring, why would you then let your money go to waste by having a website that looks terrible on mobile devices? Not to mention the countless additional benefits of having a responsive website – if customers are happy reading your content on a mobile device, they will be more likely to stay, interact with you further and buy your products and services. Isn't that the sole reason you have a website in the first place?