You can read the official announcement from Google here:
Official Google Webmaster Blog
Also, you can test to see if your website meets Google's mobile-responsive criteria using Google's own Mobile Testing Tool.
Years ago I switched all of my websites to WordPress, so personally this is not an issue for me, as I use mobile-responsive themes, but unfortunately I have one non-mobile responsive website that has been consistently earning me a decent AdSense revenue every month for the past 10 years, and I am currently too busy to attend to this site, so I will be monitoring it over the next few months to see how the new changes affect it until I can get around to addressing the issue.
I also noticed that the Warrior Forum site itself does not pass Google's mobile-responsive test, so I will be interested to see how the WF handles this new change, as I imagine this is a big deal for the site owners and for everyone who posts content on this site.
What about you? Are your sites mobile-responsive? If not, what's your plan for complying with Google's new changes? What are the challenges you are facing? Have you noticed any changes on your sites yet?
I look forward to reading your contributions.