If your site is not fully optimized by April 21st, your ranking in Google's search results will drop and your site may not show up at all. Google announced on February 26th that they will be using mobile friendliness as a major factor in site ranking making it easier for users to get search results that are optimized for their device.
If you are not sure if your site is fully optimized for mobile, you can use Google's Webmaster tools to find out. Their Mobile-Friendly Test allows you to test your site pages to see if they have a mobile friendly design and their Usability Report will give you a full list of usability issues throughout your site.
If you find that your site is not mobile friendly, check out this guide from Google Developers to learn how to optimize your site.
You can learn more about the new ranking system in the full announcement here