When a two or three dimensional wave moves past an obstacle that is in its way, it does something unexpected. It spreads out to fill in the space behind the obstacle. This behavior is called diffraction.

In the following video, the wave that is created on the right side of the frame passes through the opening in the barrier and spreads out to fill in the spaced behind it.

The little bit of the wave that makes it through is moving up and down and acts like a pebble being thrown into the water. It makes circular ripples on the other side.