A void is empty space, nothingness, zero, zilch. A place that's void of all life forms has no sign of animals, plants, or people.
You may recognize void from the Old Testament passage describing creation: "The earth was without form and void, and darkness was over the face of the deep." In other words, nothing was there: pure emptiness. When you void something or make it void, you make it legally invalid, and that kind of void often goes with null. You might tell Cinderella, "If you're not back by midnight, that arrangement with the pumpkin and the mice is null and void."