Google announced they are going to boost the effects of the mobile friendly algorithm which launched April 2015.
Google stated in the update that "it increases the effect of the (mobile friendly) ranking signal." Google said if you are already mobile-friendly, you have no reason to worry as this update will not impact you.
When the update happens it will do so gradually, so you should not see a major drop-off on non mobile friendly websites when the algorithm is rolled out. It is unclear if this rollout will take week or months, Google simply stated it would "start rolling out"" at the start of May.
This algorithm is a page by page signal, so it can take time for Google to assess each page, this is why it will be a gradual process. Depending on how fast Google crawls and indexes all of the pages of your site, the impact can be slow to show up.
If you are not mobile-friendly, or if you want to ensure you are - use Google's free tool and check their guidelines. If this is something you need further assistance with, please get it touch with the 9xb team.