What is the concept of object permanence in relation to child development?

Recognition of object permanence, as an infant's head will follow a ball as it rolls behind an obstacle and 'look' for it when it does not roll out as 'expected', indicates a cognitive stage capable of hypothesis-building: where did the ball go?

The idea is that at a certain point, the child moves from object impermanence (when he can't see something it no longer exists for him) to object permanence (even when something is not present, it still exists). That mental transition for a child affects a lot of things, such as remembering where a favored toy was left.