Mobile-first is pretty self-explanatory.
Google will look at the mobile version of your website before your desktop version, and give substantially more weight to the former.
A fully operational mobile platform — one that meets Google’s specific mobile-friendly standards has quickly become one of the most important ranking factors.
Mobile use is growing at a rate far faster than desktop use, and Google’s algorithm is already accounting for this ongoing trend by focusing far greater attention on the mobile versions of websites rather than the desktop versions.
In fact, Google’s crawling efforts already predominantly focus on mobile over desktop, further underscoring the overall importance of optimizing the mobile user experience.
In addition to the greater focus placed on the availability of a mobile-friendly version of a website, the page’s overall UX is also critical, with a particular emphasis being placed on the loading speed of the mobile-friendly site.
Given the increasing role of mobile UX as a ranking factor, it is quite likely that future versions of search engine algorithms will reward additional forms of mobile optimization.