You have a quantum particle (say, an electron) approaching a rectangular energy barrier — think of this like a hill in the middle of a flat road. The particle’s energy (E) is less than the barrier height (V₀). Classically, the particle wouldn’t be able to pass — it doesn’t have enough energy to get over the hill. But quantum mechanically, it can “tunnel” through the barrier and appear on the other side with some (reduced) probability. Top Panel: Probability Density This shows |ψ(x)|², the square