Zone of Proximal Development is the range of tasks that are too difficult for a person to learn alone, but can be learned with guidance from someone with experience in the task. This term was developed by Vygotsky as part of his theory on child development.
The zone has a lower limit: things that are difficult but possible for a child to learn alone, and an upper limit: things that a child cannot yet learn, even with assistance from an adult. For a toddler, an example of the lower limit might be putting shaped pegs into similarly shaped holes. An example of the upper limit might be adding and subracting two-digit numbers.