What is "quantum well"

n. a potential well that confines particles in one dimension, forcing them to occupy a planar region

A quantum well is a potential well with only discrete energy values.

One technology to create a quantum well is to confine particles, which were originally free to move in three dimensions, to two dimensions, by forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable to the de Broglie wavelength of the carriers (generally electrons and holes), leading to energy levels called "energy subbands", i.e., the carriers can only have discrete energy values.