“We must evolve our security solutions to meet unique customer needs at home and work by bringing together existing technologies in a new way. That is why we are introducing Microsoft Defender for individuals. It was built on our Microsoft Defender for Endpoint technology, leveraging the same trusted security that enterprises rely on,” Microsoft explains.
Microsoft Defender dashboard in Windows 10
To put it in plain English, Microsoft Defender is partially a dashboard to view and manage security protections on multiple devices for multiple people. It doesn't necessarily require that you or anyone in your family use Windows Security, either—you can view existing AV protection such as Norton or McAfee, or whatever else, and will yield to those programs if they're installed.
Since this requires a Microsoft 365 subscription, there is a cost involved. The app itself is a free add-on, but a Microsoft 365 Personal subscription runs $6.99 per month, while the Family option runs $9.99 per month.