The problem occurs when the actual mobile version of the page is vastly different from the desktop version page. To avoid such disappointments for mobile searchers, Google is experimenting with adopting a mobile first indexing strategy. In time, this will mean that the Google will primarily make use of the mobile version of a site’s content when deciding search result rankings. This will result in businesses having to adopt a mobile first strategy.
In simpler terms “Mobile first indexing is an indication that Google will consider as an overall mobile first approach when it comes to crawling and indexing the site.” An official announcement by Google can be seen here.