What is Vygotsky's concept of the zone of proximal development?

Vygotsky's idea of the Zone of Proximal Development (ZPD) is that children are able to do things that exceed their current capabilities when they are helped by someone who is more capable (usually someone older than them, like a parent). However, there is a restricted range (zone) of performance of which they are capable, even when they have help from others.

ex. A 1 year old child is not capable of finishing a 20-piece puzzle by himself. When his older sister, who is 10 years old and has finished the same puzzle many times before, gives him some help, he is able to finish it. However, if the child were to try to finish a 5000-piece puzzle, he would not be able to finish it even if his grandfather, who is a puzzle expert, was helping him.