Mobile usability has been a Google ranking factor for years, but the algorithm was primarily focused on desktop versions of sites when indexing. Now, however, the mobile-first index is officially rolling out following a promise by Google in 2016.
Put simply, this means that Google's algorithm will now view the mobile version of your site before anything else when making indexing and ranking choices. For sites that (wisely) use responsive design instead of a separate mobile version, Google will view your site through the lens of a mobile device, at least initially. Here's what you need to know: