Supposedly it is an advanced AI that will make each play-through unique by spawning zombies at different times and places, and placing pick-up's just where they are needed the most.
In reality the exact same effect can be achieved by simply randomising the activity of zombie spawn-points
Left 4 Dead 2 featured the new and improved AI Director 2.0 With new features such as apawning more zombies and weapons, and blocking off parts of the level forcing players to play the game it wants.
Bill: Damn, the AI Director placed a witch on our path, looks like we'll have to kill her to get through
Lois: Doesn't matter man! It placed PILLS HERE