Diffraction: The changing of direction of waves to bend around corners and spread as they encounter obstracles.
In the animation we show waves moving vertically upward to encounter a single slit in a barrier. (You can imagine that the lines representing the waves are the crests of the waves; troughs are halfway between the crests.) When the waves encounter the slit, they "diffract" as they come through the slit and move upward into the space behind the barrier.