Nit: I don't like the logo at the bottom near the link to the scuttle project, it is like a big red centralised node of power in the middle with a big target on it, what your trying to get us away from, probably just a me thing
Those concentric rings are a depiction of network hops. It's a smaller version of a section of an infographic that I can't find at the moment.
I agree it's not obvious.
In ssb, by default, you get content from friends, and friends of friends. You don't follow the tree of connections indefinitely.