In food idioms, "the void" is the emptiness of the stomach, both literal and the "feeling" of hunger. Food that "fills the void" satisfies raw hunger - but does nothing else. It is not necessarily tasty, but at the very least, does not taste awful enough to be outright rejected. Ex.: "So how was the meal you had at that restaurant?" "It filled the void." "...That's not much of a compliment!" <- For an expensive restaurant meal to only "fill the void" would be taken as an understated insult towards the restaurant.