When work is easy, learners can do the work on their own without any help. It is their "comfort zone." If all the work a learner is asked to do is always in the comfort zone, no learning will take place. In fact, a learner will eventually lose interest. When the work is too hard, on the other hand, the learner becomes frustrated. Even with help, learners in the "frustration zone" are likely to give up.
The area between the comfort zone and the frustration zone is the one where learning will take place. It is the area where a learner will need some help or will need to work hard to understand the concept or complete the task. This is the zone proximal development. A learner is neither bored nor frustrated, but appropriately challenged.