Starting April 21, 2015, Google Search will be expanding its use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in Google Search results. Users will find it easier to get relevant, high quality search results optimized for their devices.

Will this change affect my desktop traffic?
No. This change will affect mobile searches in all languages worldwide.

How can I check if my site is mobile-friendly?
You can check your site by testing your pages with the Mobile-Friendly Test tool. This tool shows how Google Search sees your pages. To get a list of mobile usability issues across your sites, log in to your Webmaster Tools account and use the Mobile Usability Report.

How can I make my site mobile-friendly?
Get started with the guide to mobile-friendly sites to build your mobile-friendly site. If you need more customized advice for publishers, you can check out the AdSense Multi-Screen Starter Guide.

My website is already mobile-friendly. Do I need to take any action?
If you already have a mobile-friendly site:

Double-check your site's pages using the Mobile-Friendly Test tool and see how Google Search sees your pages.
Check out the AdSense Multi-screen Implementation Guide to learn about growing audience and revenue from your site.
Use the Mobile Search Engine Optimization guide to help your site show up in search results.
Go mobile today to provide your visitors a good user experience and to make most of your mobile traffic.


Source: Google