Think of the zone of proximal development as the tasks that you can complete with some assistance but cannot yet complete by yourself. The person providing the assistance may be a parent or instructor, but Vygotsky also believed that peer interaction is an important part of learning.
The zone of proximal development is similar to the scaffolding used when constructing a building. At first, a lot of scaffolding is required, but as the building gets stronger, the scaffolding is gradually removed. Eventually the building can stand unaided.
When a child is learning a new task, for example riding a bike, he needs a lot of assistance at first. A parent may have to hold onto the child as he practices pushing the pedals, but eventually the child progresses to using training wheels. At this point, the assistance scaffolding is still in place, and the child is in the zone of proximal development. Eventually, the child learns how to ride with no assistance; all of the scaffolding can be removed, and the child leaves the zone.
The zone of proximal development is always changing as children continue to learn and gain independence. To make learning effective, teachers and peers can demonstrate the task, maintain the learner's interest and vary the amount of assistance based on the learner's performance.