This refers to Vygotsky's theory of development. He emphasized the role of society in how children learn and develop. When there are tasks to be learned, these are not seen as quickly mastered. The term “proximal” refers those skills that the learner is “close” to mastering, and the zone of proximal development is that area where the child shows growing proficiency, may need adult or societal help, but is clearly going to achieve mastery before long.
Vygotsky felt that when a child is at this stage giving just that small degree of help, or boost, or seeing that a more able peer might do this, will push the child with emergent skills towards mastery. These days, SATs will state 'working towards' when a child has not achieved a level, and a tinge of Vygotsy-thought can be detected from this.