Mobile-First Indexing is one of google's latest Mobile app development in an attempt to make mobile more user-friendly.
Here's a basic idea about what it is and how it may affect you.
WHAT IS MOBILE FIRST INDEXING?
On 1st July 2019, google launched Mobile-First Indexing. Mobile-First Indexing means that google mainly uses the mobile version of the content for indexing and ranking. Moreover, it means that google devised and ranked it's search listings based on the mobile version of the content, even for listing that are shown to desktop users. Mobile-First Indexing is about how content is accumulated rather than how it is ranked. Mobile-First Indexing is enabled by default for all new websites. And as for older and previously existing websites, google continues to monitor and evaluate pages based on best practices.
You can rephrase "mobile-first", as a reference to the fact that the mobile version will be considered the primary version of your website. It should be kept in mind that previously, desktop's version was considered as " Primary" and the mobile's version was an "alternative" version for specific cases. This was the reason for Google to motivate the webmasters with a separate mobile site to implement switchboard tags.
WHAT SHOULD BE DONE?!
Don't panic! Google's guide states that if your website is responsive and as long as your mobile and desktop versions are identical enough, you won't be needed to do anything differently.
If you have a separate mobile site, you need to make sure that the content uploaded on the website possesses high quality and the formats you use are crawlable and indexable. Also, make sure your host servers can handle an increased crawl rate.
RANKING IMPACT
If there is no mobile version for your website, google determines ranking using the desktop sites only. Although it is preferred to have a separate mobile version for better rankings, because not having a mobile version may also result in a poorer user experience than other sites. There is no official statement by Google regarding this issue.

