I'd also like a way to filter out certain communities, I'm sure that's already on the todo list.
And yes, the 4 people who run Beehaw are choosing what communities you can interact with by defederating other instances, this means people from those instances can't comment on beehaw and you won't see posts from them.
Reddit admins did something similar when they banned subreddits, except the difference is that on lemmy if you liked those banned communities then you could create an account on their instance and continue participating in those communities.
The idea is for instance admins to curate an experience for their users by choosing which instances they federate with. That's why beehaw has defederated from around 200 instances https://beehaw.org/instances while my instance has defederated from 4 https://partizle.com/instances