Google goes on to say, ..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.
Mobile-first indexing reports MOZ bloggers is exactly what it sounds like. It just means that the mobile version of your website becomes the starting point for what Google includes in their index, and the baseline for how they determine rankings. If you monitor crawlbot traffic to your site, you may see an increase in traffic from Smartphone Googlebot, and the cached versions of pages will usually be the mobile version of the page.
This represent a fundamental reversal in the way Google is thinking about your website content and how to prioritize crawling and indexation. Up until now the desktop site was considered the primary version (similar to a canonical URL) and the mobile site was treated as an “alternate” version for a particular use case.
There’s no need to panic! All our website are mobile friendly and responsive and the change is only just starting. Here's why -
According to Google’s own latest guidance on the topic, if your website is responsive or otherwise identical in its desktop and mobile versions, you probably don't have to anything differently.
Although we'll be checking our responsive websites, to ensure that mobile page speed and load time are prioritized and that images and other (potentially) dynamic elements are optimized correctly for the mobile experience.
To echo Google's recap -