This is an amazing demonstration of how sound vibrations work.
Sound is the propagation of waves through a particular medium. In this video the mallet drawn across the metal plate causes sound vibrations, which move the sand across the table.
As the waves are trapped within the table, they cannot travel anywhere and destructive interference occurs between the waves coming from different directions. The waves ‘battle’ against each other until they reach a perfect harmonisation.
This sort of ‘trapped’ wave is called a standing wave and they are exactly the kind of wave you see when you wiggle a skipping rope from either end and form a single pattern with peaks (where the rope is at the highest point) and nodes (where the rope appears to stop moving).
The places where the coloured sand collects in this video are the nodes of the wave — places where there appears to be no movement.
The anti-nodes are the opposite of the nodes. They are the points where the sand is moving away due to the plate vibrating up and down.