Google will be giving priority to the mobile friendly websites on their ranking on their search results. Such changes are rolling out from 21st April 2015. According to Google Webmaster Central Blog, it is to make search results high-quality and relevent. For this text have to be readable without tapping or zooming, tap targets are spaced appropriately and the page avoids horizontal scrolling.
This update will affect only search rankings on mobile devices of all languages and will apply to individual pages, not entire websites.
If your website pages are not mobile friendly, you might face significant decrease in mobile traffic from Google search. To check if your site is mobile friendly or not you can use Mobile-Friendly Test or you can view the reports on Mobile Usability report in Webmaster Tools. Once they become mobile friendly, your site will be automatically indexed again on Google mobile search. Or you can speed up the re-indexing process with Fetch as Google with Submit to Index, and then your pages can be treated as mobile-friendly in ranking.