Google began rolling out mobile-first indexing on March 26. Here's how they described the impact of the change on the Google Webmaster Central Blog:
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.
We continue to have one single index that we use for serving search results. We do not have a “mobile-first index” that’s separate from our main index. Historically, the desktop version was indexed, but increasingly, we will be using the mobile versions of content.
Now if you just read that and thought: "Ack, SEO things, I'm not sure what this actually means for me!" You're in luck.
The smart SEO folks over at Seer wrote a helpful post explaining what they thought were the most relevant portions of this important announcement.
The bottom line here is this: if you have a mobile responsive site you're in the clear. But, if you either have a separate mobile site or your site serves different content based on the user's device, you have some work to do. Once Google rolls out mobile-first indexing to your site it would rank the mobile version regardless of whether it has less content than the desktop version.
So, what do you do? Our first recommendation would be to go responsive, then you have a single code base for all devices. Our next best suggestion, because not everyone can afford to jump into a responsive design right away, is to make sure your content is the same for both desktop and mobile.
If you need help with any of these issues please reach out. I'm happy to talk.