What Is Meant By A Mobile First Index?
Simply put, Google maintains two separate sets of search results based on the device you are conducting your search from so you will either see search results tailored for your laptop/desktop or you will see search results Google feels are more appropriate to view on a mobile or tablet device.
As usage has migrated towards mobile devices Google has continued to modify their mobile ranking based on a number of factors related to how user friendly a website is on mobile devices but also how quickly a site loads on smartphones. Google has slowly integrated "mobile friendliness" into everyone's vocabulary all in preparation for this one single gigantic shift in search.
Google announced late last year that sometime in the first quarter of 2017 they will use the mobile version of indexed search results as the primary results shown on all devices.
Just think about that for a second...
In terms of SEO this is the single largest change since Google added the Penguin update to their ranking algorithm. Suddenly mobile SEO will be the Holy Grail for any website competing in the search results. Most Atlanta area small businesses barely have their heads wrapped around SEO let alone how to SEO for mobile results. One thing you can count on is page load speed is going to be a critical factor in how well your site will rank in the mobile index. I am providing two links below so you can test mobile page load speed and also AMP markup on your site.