The Idle state is ultimately the starting state for a zombie, and a passive state. Normally, an NPC in Idle will stand around and just look about. It's a state from which action may begin. For Dead Keys, the zombies remain in Idle until instructed to change, based on camera movement and the position of the player:

As the player enters the Chase range of an NPC, the NPC comes to life. In many games, a deciding factor will be the line of sight. The enemy chases, or pursues, the player as they enter their line of sight. However, for Dead Keys, this is unnecessary because camera movement, as the player progresses from one point to the next, determines whether an NPC moves into view. For this reason, the StateIdle coroutine will remain almost empty. However, the SetInteger function will be called to set the animator's integer parameter...