As of April 21, 2015 Google will be expanding its usage of mobile-friendly factors in its search algorithm. This change will affect mobile searches in all languages worldwide and will have a significant impact in the Google search results.

Anyone who does not have a responsive site capable of serving multiple devices will be affected... but, here are the things they are looking for in particular:

A site that has software that is not common on mobile devices, like Flash

A site that uses text that is not readable without zooming

A site that doesn’t size content to the screen so users don't have to scroll horizontally or zoom

A site that doesn’t place links far enough apart so that the correct one can be easily tapped

Read more here: Google Webmaster Central

Test your site now: Google Mobile Friendly Test

Responsive website design is an approach to web design allowing creation of sites with an optimal viewing experience across a wide range of different devices such as desktops, tablets and smart phones. Before responsive web design, you needed a different version of your website for each device.