First rolled out in 2018, mobile first indexing is a new way for Google to crawl, index and rank websites. In order to provide users with relevant results, Google needs to find out more about your webpages. To do this, Googlebot ‘crawls’ through various site information, including data from past crawls, sitemaps, dead links and new content. With this information, Google can then index and rank websites in order to provide users with relevant SERPs.
Prior to mobile first indexing (MFI), search engines relied predominantly on the desktop version of a web page for crawling, indexing and searching. This means that the desktop version of your site was the main factor when it came to your SEO rankings.
Since the introduction of MFI, however, Google places more importance on the mobile version of webpages. Due to this, it’s the mobile version of your site which will have the most impact on your rankings.