What does this mean?
This is what Google said on their Googleblog:
To recap, our crawling, indexing and ranking systems have typically used the desktop version of a page’s content, which may cause issues for mobile searchers when that version is vastly different from the mobile version. Mobile-first indexing means that we’ll use the mobile version of the page for indexing and ranking, to better help our – primarily mobile – users find what they’re looking for.
There is no argument at all that this will not improve the experience for mobile users, which as we surely know by now is becoming the majority in most areas.
Google will now crawl mobile webpages first and where one is not found will use the normal version as seen on a desktop.
Almost 2 years ago when this was first alluded to, their was a panic amongst webmasters, site owners and general businesses that this meant that IF you did NOT have a fantastic mobile-specific page then you were basically going to be thrown out with the garbage.
You can breathe a sigh of relief as this will NOT happen.
However, you should still concentrate on having a site that offers a great mobile experience. Typically bad examples of this are the older sites based on fixed 'boxes' of content which are all but unreadable by somebody on a mobile phone. The opposite of this is the more modern responsive website that changes its format as the viewing device gets smaller. These should not be affected by this at all.
Therefore, if you do have excellent content for mobile phones then you can expect to see a rise in that content in the search engine rankings for the same key search phrases.
If you follow the best practices for mobile phone content and have this entered seperately into the Google Search Console then you should be fine.
A good link to check out is here:
Google will notify you if your site has been enabled for Mobile-first indexing.