Turns out it's not a super esoteric concept. It is a "map of where the electrons are", but (and of course the uni PR won't tell you this) it's not a literal position map, it's a map in momentum space. And like you say, it's focused on the highest energy electrons. So it's really saying how the highest energy electrons are distributed in momentum space.
I read that page before posting. It's pretty dense. :) Eventually I got to the "map in momentum space" part and it started making more sense, but I still had to read the page about energy bands a couple times, and it's still pretty hazy.
Turns out it's not a super esoteric concept. It is a "map of where the electrons are", but (and of course the uni PR won't tell you this) it's not a literal position map, it's a map in momentum space. And like you say, it's focused on the highest energy electrons. So it's really saying how the highest energy electrons are distributed in momentum space.