The "Zone of Proximal Development" is a theory made popular by Vygotsky, a psychologist. The Zone of Proximal Development is a theory that talks about the sweet spot of learning. This means that students should be in the perfect proximity with their instructors in order to have optimal learning. Below is a figure of what the zones of proximal development looks like:

The sweet spot is the middle circle where the student is able to work to their fullest potential with help or guidance. A common way this is used in the classroom is through scaffolding because there are different levels of supports intended for students to be in their sweet spot of learning.

Resources:

https://www.sciencedirect.com/topics/psychology/zone-of-proximal-development