On April 21st, Google made a major update to their search algorithm. Their algorithms are tweaked and changed all the time, but this update is significant because it totally changes the way Google indexes non-responsive websites.
If your website isn't considered mobile-friendly (more on that in a moment), it will lose rankings on google when a user is searching from a mobile browser. Although the changes are in place, the results could take days, weeks, or months to take. According to Google, this change doesn't affect desktop search. It affects mobile devices like smartphones and tablets (however Windows 8.1 tablets are only affected when using the browser from the Metro interface).