The x axis is position. The y axis is energy. The blue box is a potential energy barrier. The red curve shows the wavefunction of a particle at a certain energy level coming in and tunneling through the wall. (the wavefunction actually live on a different y-scale from this plot and is only superimposed here for illustrative purpose, so don’t use the energy y-scale to read into the amplitude of the oscillatory part).

more info: https://en.m.wikipedia.org/wiki/Quantum_tunnelling