Google has officially concluded the testing phase for mobile-first indexing in its search service, and is now starting to roll out the new indexing method to some sites that the search giant thinks have done a particularly good job in following its established best practices for a mobile-friendly page. Affected websites will not see their rankings change among searches conducted from desktop devices, game consoles, and other oddball fare, but mobile phones, tablets, and other types of devices that use a mobile processor and ask for a mobile interface on the web will start seeing these sites above other comparable results that are not deemed to be mobile-friendly.
Google uses a special version of Googlebot that emulates a smartphone in order to crawl the web from a mobile point of view, and the bot is able to figure out just how mobile-friendly each page is by comparing it to a set of standards that Google established. Pages that get into mobile-first indexing will see more traffic from that Googlebot as it continually checks to ensure that the content and layout of the page is still compliant with mobile-friendly standards. Getting into this ranking does not necessarily mean that a given site will actually outperform comparable results in all searches; Google uses a number of signals and metrics to determine a site's relevance to a given search, and a wide range of other factors could outweigh a site being mobile-friendly.
This long-awaited rollout is part of Google's ongoing efforts to streamline the search and web experiences for mobile users, who are often whipping out their phone to look something up in a hurry in the heat of a moment and have limited time to spend sorting through results or dealing with pages that load slowly or don't work well on a mobile device. The rollout is only going to affect a limited number of the most standards-compliant sites at first, and will roll out more widely and begin branching out to sites that stick less strictly to Google's mobile-friendly guidelines in the near future. Webmasters should take note that this does not affect the rankings bonus that you will get for your page loading fast and having a good amount of its content upfront, where it's easy for users to find on any platform. Instead, this new boost stacks with that one, which means that a very mobile-friendly page that loads quickly and has a wealth of content presented to the user from the start has a high chance of being the first thing somebody on a phone sees when making a relevant search.