Since 2016, mobiles have become the predominant medium for online activity, whereas before the majority of internet use was done on desktop and laptops. At first, Google had separate algorithms for both types of searches, their desktop algorithm catering to those surfing from full size devices, and their mobile algorithm for those on smartphones or tablets. Though these algorithms had the same core functions, they were both fine tuned for different details, such as the mobile algorithm focusing more on load speed than the desktop algorithm.
With the majority of search queries now being done from mobile devices, it makes sense that Google would spare their resources in running two separate core algorithms and simply focus on the most used one.
The mobile algorithm take-over is something that has been coming for a long time, ever since 2014 when the mobile algorithm began and Google started giving priority in searches to responsive websites that were mobile friendly. Now, website development is most often based around a mobile design, with the desktop website version being secondary. Websites are being forced to become faster loading, and use image compression to help reduce bandwidth use for mobile users.
So what does Mobile First Indexing mean?
Mobile first indexing means that the index copy of the website that Google will now be keeping is the mobile version. So if there is content that is “hidden” from mobile devices using a “display:none” tag or something like that, it may not be helpful anymore for SEO. If your website is already mobile friendly, then there is nothing to really worry about, but if you still have a static website then you should seriously be considering finally moving to a mobile friendly platform.