What's the difference between mobile-first indexing and mobile-friendly SEO?

Mobile-first indexing means Google predominantly uses the mobile version of the content for indexing and ranking. ... Google Search continues to show the URL that is the most appropriate to users (whether it's a desktop or mobile URL) in Search results.

Mobile SEO refers to the search engine optimization of websites combined with flawless viewing on mobile devices, such as smartphones and tablets. Thanks to the increasing boom of portable devices, webmasters should be highly concerned with their mobile SEO plan. ... Google is already favouring mobile friendly sites.