Google plus introduced "circles" as a basic structure for social networking, based on the idea that your social network is actually a bunch of different networks, all connected to one You, sometimes overlapping.
With multiple instances and the right clients to manage them (e.g with Subway Tooter) Mastodon says that there's more than one You, only connected to one another if you want, and each You can have its own social network, sometimes overlapping.
#thoughts