Mobile-first indexing means that Googlebot will now use the mobile version of your site for indexing and ranking, to better help our (primarily mobile) users find what they’re looking for.

Google’s crawling, indexing, and ranking systems have historically used the desktop version of your site’s content, which can cause issues for mobile searchers when the desktop version differs from the mobile version. Google says “Our analysis indicates that the mobile and desktop versions of your site are comparable”.

So what does this mean? Google has now put your mobile device completely in the front seat and your desktop/laptop computers in the backseat. As far as Google is concerned, your mobile device has the primary version of your website and the desktop is the alternative version.