Since 2012, Google has been testing mobile-friendly labels in the search results. Most recently, Google ramped up those tests by testing mobile icons, not mobile friendly icons and text based mobile friendly labels. Last night, Google started rolling out the mobile-friendly text labels in the mobile search results.
If you pull out your smartphone and search for something in Google, you will probably see some results labeled as mobile friendly.
For example, this site comes up as mobile friendly:
Why does it show for this site? Well, it passed their new mobile friendly test, and if your site passes it, it should also show the mobile-friendly icon:
For example, my corporate site works also:
What does it look like when a site is not mobile friendly? Well, doesn't look like much, there is no warning, just no added label about it.
This site did not pass:
So use this new tool, check your mobile usability reports in Google Webmaster Tools and make your site mobile friendly. It will help with conversions and now possibly clicks from Google's mobile search results.