Up until now Google has used one index to generate the response to all queries in the Google search engine, regardless of whether the query was on a desktop or mobile platform. Now Google has announced that it will, within the next few months, start to use a separate index for searches on mobile devices. It has also said that the mobile index would become the primary index, although didn't provide further details as to what that would mean in practice.
Historically there had always been more searches conducted in Google on desktops than on mobile devices. That changed during 2015, as Google confirmed that the quantity of searches on mobile devices had surpassed desktop searches for the first time. In April 2015, Google also updated its ranking algorithm to reward sites that had been optimized for mobile use. Most web sites that weren't classified as mobile-friendly were immediately ranked lower in the Google search results.
Introducing an entirely new index for mobile opens up a range of possibilities, such as the fact that the new index could contain only mobile-friendly sites. It seems likely that if the mobile index becomes the primary index, then it will become the priority for updates and be kept more current than the desktop index. Although there are now more searches on mobile devices there are still a huge number on desktop too, and it would cause some frustration if the desktop search updates became less frequent, especially for site owners that still depend more on desktop users.
Google also faces more challenging competition on mobile platforms, especially with the likelihood that verbal queries to personal assistants will become increasingly common. Both Amazon's Alexa and Apple's Siri use the Bing search engine owned by Microsoft as their default search tool, although both can access Google search too. Google will want to ensure that it can still provide the best search results, and a new mobile index probably is the best way for it to achieve that. At the moment for example, data from a desktop version of a site can influence the ranking of the mobile version. For website designers and owners, having a site that works well on a mobile device will become even more critical.