Google has taken the step to explain more about Mobile First indexing which is aimed at mobile SERPs. It explains it's being cautious about rolling this out, while closely monitoring the results. It's evaluating sites for mobile-first indexing readiness, and it'll transition the sites to the index when it feels the sites are ready. Check out the six key points if you want your site considered for the mobile-first index.
As we all know, Google is specifically looking for mobile content to serve in mobile SERPs, and if you have content that's appropriate you start to see increased crawling by Smartphone Googlebot
[support.google.com] along with the snippets in the pages for the mobile versions of the pages.
There are six points Google makes, which most of us already know, but here's a quick reminder. Mobile site should also have high quality content.
Structured data should be on both mobile and desktop versions of the site.
Metadata should be present on both versions of the site.
No changes are necessary for interlinking with separate mobile URLs (m.-dot sites)
Check hreflang links on separate mobile URLs.
Ensure the servers hosting the site have enough capacity to handle potentially increased crawl rate.