It was announced at PubCon (a webmaster conference) by Gary Illyes that Google is planning to roll more sites into the mobile-first index over the next several weeks.
The following tweet was posted by Marie Haynes, who was in the crowd, on Twitter:
— Marie Haynes (@Marie_Haynes) February 21, 2018
Jesse McDonald, who was also in the crowd, posted this on Twitter as well:
— Jesse McDonald (@jesseseogeek) February 21, 2018
Google’s mobile-first index is their attempt at looking at the web and indexing and ranking the web from a mobile-first perspective. So if you have a mobile-friendly website, rather than indexing the desktop version, Google will index that website’s mobile version first.
There is a large FAQ for everything we know about the Google mobile-first index on Search Engine Land.