Chladni figures are patterns that appear on surfaces when they are made to vibrate at certain resonances. It sounds dry, but throw in some sand and just the right frequencies and it becomes an amazing video art project.
Short burst or single waves of many sizes can travel through a surface, but only a few can set up shop there. Depending on where a surface is anchored, its size, material, and thickness allows it to resonate at certain frequencies. Resonation is a state at which the waves traveling back and forth achieve a stable structure. Some parts of them build on each other. Some parts of them diminish each other, but they don't interfere which each other enough that they choke themselves out.
In these stable states, certain parts of the surface will be vibrating massively. Others will be still. If a grain of sand were placed on one of the highly vibrating points, it would get bounced away, collecting in a still point. Different resonances will produce different spots of movement and stillness, jetting sand around a surface into different patterns each time the resonance frequency changes. Each of these patterns forms what's called Chladni Figure. They can be incredibly beautiful, especially if you throw in some rainbow sand, as they do in this clip.