Until now, when Google trawls the internet creating its vast index, it looked at the desktop version of your website. But that is changing. So you’ll need to ensure your mobile site is tweaked in order to maintain your page ranking.
Google says “Mobile-first indexing means that we'll use the mobile version of the content for indexing and ranking, to better help our – primarily mobile – users find what they're looking for”.
If your mobile and desktop sites are different, users can get a different experience today and see search results that aren't reflective of the expected content. Given mobile internet use is becoming the prevalent way of accessing the web, it makes sense to use excerpts from mobile, rather that desktop, sites.
Responsive sites are already prioritised in search results so this step isn’t unanticipated. Google has provided some advice on how to prepare for this change including ensuring your mobile site contains your most important content and both versions of your site contain the correct metadata.