Although mobile responsive websites and mobile websites sound similar, there is a difference between the two. A mobile responsive website is a duplicate of your desktop website that has been updated to be responsive or “friendly” on mobile devices. The design and content conform to a specific viewport size of the device you are on. This means that the duplicate website can adapt to a multitude of devices and their different screen sizes. A mobile website is a completely separate website with different content and could be or might not be responsive to the device you are using.
“Our crawling, indexing, and ranking systems have typically used the desktop version of a page’s content, which may cause issues for mobile searchers when that version is vastly different from the mobile version,” wrote software engineer Fan Zhang. “Mobile-first indexing means that we’ll use the mobile version of the page for indexing and ranking, to better help our—primarily mobile—users find what they’re looking for.”