The zone of proximal development is the difference between what you can do without help and what you can do with help. By doing something just above your ability, you come to learn. The theory of the zone of proximal development originates from Lev Vygotsky (1896-1934). Everything you can do independently is the actual development. Everything you need help with is the proximal development. The area in between is the zone of proximal development: activities that you cannot yet do independently, but can do with social support during their execution. You are addressed in your zone of proximal development by (supported) offerings that are just above your level: not too easy, challenging, but not too difficult. In doing so, the support is gradually reduced at each level until someone can do it themselves. That is the moment the student has reached the new level. This is also called scaffolding.
Smit, J. & van Eerde, D. (2013). What counts as evidence for the long-term realisation of whole-class scaffolding? Learning, Culture and Social Interaction, 2(1), 22-31. doi: 10.1016/j.lcsi.2012.12.006