According to google, to make search more useful, the have decided to make their indexing mobile first.
Their algorithm will use the mobile version of websites to index search results. This is a major change and something website owners and bloggers need to understand it.
Why is Google doing this?
Because mobile search has surpassed desktop search and more people access Google via their phones than on desktop computers.
So, it makes sense that mobile becomes the dominant index.
Does your website have to be designed and coded as mobile-first?
No. It means you should make sure the mobile version of your website presents the same content as the desktop version.
What if I only have a desktop website?
According to google you are ok, but I this as a false sense of security for website owners.
If Google is positioning mobile as the dominant version and search authority, then desktop version will not compete with a mobile version. You need to be well positioned for mobile users.
If you have a mobile website and a separate desktop website
Google has not clearly addressed this issue. It could alter your ranking since SEO success is based on link building into specific URLs.
It is safer to get a mobile responsive website.
Wrap Up Summary
Business owners will have to stop ignoring mobile users. Mobile users are your dominant users and websites will have to be tailored towards them.