Goo is the resource used to pay the cost of cards during a game of Zombie Battleground. A card's goo cost is printed on the top right corner of the card, and a player must have that much goo available in order to play it.
At the start of the game, both players begin with a single bottle of goo. At the beginning of each turn from that point forward, players gain one more goo bottle automatically, and any goo bottles that were emptied by playing cards on previous turns are refilled.
This means that in a normal game a player will have 1 goo to use on turn 1, 2 goo on turn 2, 3 on turn 3, and so forth until reaching 10 bottles of goo.
Once a player has reached 10 bottles of goo, they no longer gain additional bottles at the start of each turn (although they do continue to be refilled as usual).
Some cards use a special keyword called overflow which temporarily gives a player extra goo to use in a turn. The overflow keyword is always accompanied by a number which indicates how much additional goo the ability will grant. Additionally, goo obtained from overflow effects can grant goo beyond the normal limit of 10 goo bottles, allowing the player to make use of particularly powerful cards which cost more than 10 goo, such as Yggdrazil or Gargantua.
Overflow abilities on zombie cards can only be activated if the zombie's faction matches the faction of the player's overlord. For example, if your deck uses a Fire faction overlord, playing the card Zeptic from the Toxic faction would not grant any additional goo. This rule holds for faction zombies which grant Tainted Goo or Corrupted Goo.