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Herd Mode was a Gamestop/ebGames pre-order bonus for the game. It's an arena mode where they throw waves of more and more zombies after you, and as you progress through the waves more parts of the map are unlocked and more weapons and supplies are dropped for you to use. There's only one map to play on and it's not multiplayer. If you want to see some video of it, search YouTube for "Walking Dead Survival Instinct Herd Mode".
Hopefully sometime in the future they'll make it available to buy, along with the Amazon pre-order bonus of five extra melee weapons to use in the Campaign.