When an undulatory phenomenon meets a small obstacle in its path it is able to go round it. This is how we are able to hear a conversation taking place on the other side of a wall.

In the same way, when wavefronts meet a small opening, once through, they propagate in all directions. 

These two types of behaviour constitute diffraction, a characteristic property of undulatory movement to such an extent that the undulatory nature of light was only admitted after it was proved that it experienced diffraction.

In the following visual we will study diffraction in its simplest aspect, when it occurs because an aquatic  wavefront finds an opening through which it can propagate.