PlayerUnknown's Battlegrounds is a frantic game of surviving against your fellow players in a wide open sandbox with scavenged equipment. The one differing factor with the game compared to comparative experiences like DayZ and H1Z1, is that there are no zombies - that is unless you play zombie mode. The different with that though, is that the zombies aren't controlled by a mindless AI, but are instead made up of melee weapon wielding naked players.
Zombie Mode pits five players against 95 zombies. Zombies are faster, come equipped with hatchets and frying pans and are gunning for the players. They initially come in ones and twos, but as the playable area gets smaller and smaller, the numbers really increase to the point where tens of naked, hungry 'zombies' are running right for you.
As Kotaku explains, although Battlegrounds is typically a more stealthy experience, Zombie Mode makes the game much more frantic. It's a near constant chase from the start of the game until the last warm or cold bodies are felled.
That means it forces strong communication between players. Teamwork on both the zombie and the player's part, but it seems to be going down well, as the above streamer is far from the only one enjoying the game mode.
Now it remains to be seen if PlayerUnknown will make this into an official game mode.