Google announced today that it is testing out mobile-first indexing. It is likely this change will become permanent once Google perfects the user experience. And it is user experience that is driving this search engine indexing change, as more web content is accessed via mobile devices.
Google’s goal is to have the most relevant search results appear for mobile users. As of now, this does not always happen, as many sites deliver different content via mobile versus desktop websites.
While this project is only in the experimental stage right now, you will want to take steps soon to ensure that your mobile content captures the same as your desktop content. The best way to do this is to have one responsive website, as the content remains the same regardless of which device a user uses. If you already have two separate sites for mobile and desktop, follow the tips from Google’s Webmaster Central blog.