In Death Stranding a Voidout is a big explosion caused when a BT (Beached Thing) consumes a living human. Then matter and antimatter mix and this creates a powerful explosion that leaves behind a big crater.
When people die they “go necro” after death. They enter a state of necrosis and turn into a BT (ghost). If the BT ghost then consumes a human, it causes a voidout. That’s why BTs are so dangerous and people live underground to avoid them.
When bodies are put on ice it can take up to 40 hours for them to turn into a BT (we learn this from Igor of Corpse Disposal during the Prologue). If they are just left rotting, they will turn into a BT within 24 hours.
Because this is a huge threat, cities have established Corpse Disposal Teams. They bring corpses to an incinerator out of town. Once the body is burned it can’t harm anyone anymore.
That’s also why Sam isn’t supposed to kill anyone in the game. If he kills a human, they would turn into a BT after a while and you’d have to bring them to an incinerator before they pop into a BT. It’s better to stun human enemies (e.g. with rubber bullets). If Sam somehow causes a voidout, he dies instantly and you are forced to reload the last save.
Simply Put: Voidout = a BT (Beached Thing) consuming a Human, thus causing an explosion and leaving behind a big crater.