What that means is this -- Googlebot will now use the mobile version of a website for indexing and ranking to better help the growing number of predominantly mobile searchers find what they are looking for on the web.
Historically the desktop version of your website content was used for indexing, but now Google is focusing on mobile content. Why? Because mobile devices accounted for 60% of all traffic in Q1 of 2018 (and growing), Google wants to make sure they are best serving their biggest set of users.
This infographic by Page1solutions shows the impact of mobile devices on Digital Sales.