I asked this in the matrix channel, I think it’s good to know for others too:

Is it somewhere documented what the difference between Hot and Active? top and new I can imagine what it means.

Another thing is the popular one, this sounds to me like hot also.

Rimu replied:

They are not documented, no.

The group on the left are sorts, which change the order of posts. The group on the right are filters, which determine which type of communities posts come from.

Hot uses votes combined with post age for ranking. Active is just the most recently commented posts (like old forums used to).

Filters: subscribed: communities you joined
local: communities on the same instance
popular: communities the instance admin has chosen to be popular. It is not automatic, it is the opinion of what the admin prefers. Use “/admin/communities” for this.
all: everything