Mobile-First Indexing

Mobile-first indexing means that Google will be using the mobile, instead of the desktop, version of webpages to index and rank URLs.

The purpose, in Google's own words, is "to better help our - primarily mobile - users find what they're looking for." Websites that are not responsive and that have different content on the mobile and desktop versions will be more likely to see their rank affected by the switch over to the mobile-first index.
Learn more about mobile-first indexing

Google algorithm