Earlier this year, Google posted that they are to boost the ranking of mobile-friendly pages. This basically means that if you are using Google on a mobile device, sites that are optimised for mobile devices will start to get higher search rankings than non-optimised sites. You can use this tool to check if your site is mobile ready.
Google do say if your site isn't optimised for mobile devices, you could notice a significant decrease in mobile traffic from a Google search. The good news is that it will be automatically re-crawled once your site has been optimised.