[bluesummers5651]

The simplest version of this problem involves a "potential barrier." Another loose classical analogy here is considering a ball rolling towards a hill. Everyone knows from experience in classical systems that with sufficient speed i.e. kinetic energy, the ball can go over the hill i.e. there is more kinetic energy in the ball when it meets the bottom of the hill than there is potential energy the ball would have at the top of the hill. If it has less energy, it will not make it past the hill. The weird thing about the analogous situation in quantum mechanics is that even if the particle has less energy than the potential barrier (the hill), it has a non-zero probability of being on the other side due to the wave function exponentially decaying in the barrier.