Google is know by it's most famous algorithm updates like Panda and Penguin but since they realized the importance of mobile devices and that the majority of internet connections are made using smartphones and tablets they are forced to make changes to show mobile friendly results.
April 21 of this year 2015 is the time you have to finish Mobile Optimization until Google rolls out this update.
According to them websites that don't comply or are not mobile-friendly will get removed from search results on such devices. Meaning that you will be loosing a ton of traffic since Google gets more than 60% of searches using mobile devices.
Unconfirmed sources say that it could even have a negative affect on the website general rankings and SEO.