Consider a particle of energy E is incident from the left on a rectangular potential barrier of height and width and on both sides of this barrier at which means that no force act upon the particle in this regions. Thus, the potential function is defined as if a particle having energy E less than then classically the particle will always be reflected completely and hence will not penetrate the barrier. But according to wave mechanics the particles has some probability of penetrating this barrier and appear in region III.
This is called quantum mechanical tunneling or tunnel effect. According to this, if a particle is moving towards the barrier with energy less than the height of potential barrier, then it is not necessary to reflect completely by the barrier but there is always a probability of particle to cross the barrier and continue its forward motion.