The shadowy rectangle in the center of the image is the shadow of a rectangular barrier placed in the water in the ripple tank. As waves pass the edge of a barrier they are diffracted: the edge acts as a point source, generating circular waves in the region to the left of the edge.

As the wave frequency is increased, the extent of diffraction is reduced. Diffraction, essentially vanishes at high frequencies. This effect is best seen by comparing images from the beginning and middle of the movie with those near the end.