Starting April 21, Google Search will be expanding its use of mobile-friendliness as a ranking signal. If your website is not mobile friendly, Google might rank it low. This is a step by Google to make its search result better by giving priority to those website which will appear good on mobile phones. A lot of users now do Google search using their phone and if the website is not mobile friendly, the user experience is hampered.
To make your website mobile friendly, simply choose a website template for your website CMS which is based on bootstrap or media queries. If you are in a real hurry then even fluid width website template will also pass mobile friendly test as it adapts to the screen size. If you are unable to convert your existing website design then you can create a separate mobile version of your website as well and redirect mobile users to it.
Double-check your sites are ready for this change by testing pages of your site with the Mobile-Friendly Test tool here.