The mobile first index was first announced by Google back in 2016.
This came about because of consumer behaviour and our love for Smart Phones.
Mobile devices account for over 57% of all of Google’s traffic, so focusing its search engine results on mobile first makes complete sense.
The move to mobile first indexing started fully last month, March 2018.
Google will now use the mobile version of your site for indexing and ranking.
As with most major algorithm updates, sites may see a sharp drop in ranking, then if the site meets the new criteria and is mobile friendly, and compatible with the new update, a steady climb back to where you started normally occurs.
If your site is not mobile friendly, you may see a drop in ranking and traffic.
If you have a separate mobile site, rather than a responsive site, the mobile site will be used for indexing rather than the desktop.
Maybe now is the time to upgrade your older, outdated, non mobile website!