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).
The red curve is real part of Psi 😉.