Generally they're referring to the thin veneer of blue and green on the surface of the planet. Things would have to get pretty heinous for humans to go extinct, but many species are already threatened.
I realize this is subjective, but to me, biodiversity is obviously far and away the greatest thing in the known universe. It's astounding we (humans in the aggregate) care so little for it.
It's like mostly poisonous gas and water, chill down!