In a recent announcement by Google, they mentioned that over half of the pages shown in search results globally are from Google indexing using mobile-first indexing procedure. That states that the pages you visit from a Google search are based on how Google crawled and indexed a particular content based on the mobile version of that page.
About Mobile – First Indexing
Mobile-first indexing is all about how Google crawls and indexes the web. Google looks at the mobile version of the page rather than looking at the desktop version of the page. Google is actually crawling and indexing your web page on the basis of how it renders on a mobile phone versus a desktop computer.
Google will surely notify you when your site has moved to mobile-first indexing within Google Search Console. It will also label your site as last crawled by the Googlebot smartphone useragent in the URL inspection tool.
It is essential to make sure to check your messages in the Google Search Console and pay attention to your structured data and alt-text for images on mobile pages with this change.
Why is it significant?
In case your desktop pages are distinct from your mobile pages both in terms of content and structured data, then your ranking might get affected. Google has largely moved sites that have uniformity between mobile and desktop pages first to mobile-first indexing. As a result of over half of the search results now indexed through mobile-first indexing, it is merely a matter of time until your web pages are moved over as well.