For a while Google have been tagging sites as 'mobile friendly' and this is indicated on search results pages on mobile devices.
But from 21st April 2015 they are taking this one step further and penalising websites that are not mobile friendly in their search results.
Their official wording is:
"Starting April 21, we will be expanding our 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 our search results. Consequently, users will find it easier to get relevant, high quality search results that are optimized for their devices." 
So, will you drop off of Google?
Well, this tool will tell you if your website is mobile friendly or not:
If you're mobile friendly, there is no need to worry.
If you're not, then there could be. Don't panic immediately; careful reading of Google's statement suggests that your rank in Google is only likely to be affected if users are searching on a mobile device, and not a desktop PC.
But this still could be bad as we find that typically on websites that we've designed, between 15% and 75% of users are on mobile devices.
The obvious next question is how this can be rectified. Web designers can often make existing websites mobile friendly, and in other cases a rebuild would be more beneficial. It all depends on various factors including how the website has been coded.
If you'd like us to look at your website, and let you know if it's mobile friendly, likely to be affected by Google, and the complexity of rectifying any issues, then please get in touch.