This is a representation of a wave function Y(x,t) which initially is a Gaussian wave packet moving to the right. As soon as the wave hits a barrier (which is indicated by the red potential), it splits into a passing and a reflecting part. In terms of physics this means that with a certain probability, a particle is able to penetrate a barrier that according to classic physics is impenetrable (tunnel effect).
The probability distribution p(x,t) = |Y(x,t)|2 is shown in black. Blue is the real part and green demonstrates the imaginary part of Y(x,t). In this applet the wave is reflected at the borders of the observed area, where it creates similar patterns as in a bathtub after some time.