The Science of the "Chladni plate"
This is a special demonstration of a device called a Chladni Plate.
When the plate is "bowed" is produces a note (usually not a very nice one!). However, if it is bowed carefully, it can produce a single quite pure note. This is a resonant pitch of the plate, which means that the wavelength of the sound waves match the size of the plate in some way (for example, the wavelength of the note might be exactly a quarter of the length of the side).
Because the waves of sound "fit" neatly onto the plate, standing waves are set up in a complicated pattern. This means that some parts of the plate vibrate a lot, but others hardly vibrate at all.
The salt moves from the vibrating bits, but stays in the stationary ones giving the patterns that you see in the video!