Background Info:

Electro Magnetic Radiation or EMR moves in waves. Anything that isn't solid matter (stone, wood, metal, glass, sand) is a part of the EMR spectrum.

Picture waves at the beach/in the ocean. The water is matter, but the waves themselves are a shape in the water generated by gravity, wind, the moon, etc. The first thing you should know is that the number of waves per-unit time is called frequency (basically difference between a buch of waves quickly or a few waves every so often, and everything in between) i.e. the frequency of the waves crashing against the shore; And the width of the waves themselves is called wavelength.

The speed of the energy (how quickly the waves are moving toward the beach) is calculated by multiplying the width of the waves times the number of waves hitting the beach in a given time, i.e. velocity equals wavelength times frequency.

Now that you have these vocab words jumbling around in your head, let's talk about the EMR spectrum:

IF we keep velocity constant, our two variables must be inversely proportional. In other words: if the waves get wider, the number of waves must go down, and vice versa.

Starting from really wide waves and really few of them, and going to really skinny waves and very many of them, the spectrum goes like this:

Radio waves, micro waves, infrared waves, visible waves (color), UV, Xray, Gamma

Notice that radio, color, X-ray, and infrared (heat), which all seem to be totally different things, are actually the exact same thing but at different levels of frequency vs wavelength: energy.

--------------------------------------------------------------------------------------------------------------------------------Sound waves:

You may have noticed that sound was not included on the spectrum. Note that radio waves are also not sound. BUT, you might already know that sound is a vibration. You may have also learned in science class that heat is also a vibration. Have you ever seen "infrared goggles" that can see heat? Think of Predator vision from The Predator movies.

Infrared is the energy representation of vibration. Because of this certain wavelengths of infrared can represent heat, and others can represent sound (although it would be difficult to see sound in infrared because you would need really high sensitivity which would blow out the optics because everything is always vibrating, but that's a whole other can of worms).

Metal is a great conductor of multiple things, but the two biggest are heat and electricity. That's why your stove top is metal, that's why pots are made of metal, and that's why wires are made of metal.

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Putting it all together:

Because metal "conducts" heat (conduction is, for our purposes, the opposite of absorption) we can infer that metal simply conducts infrared very well. The particles in metal are all very close together and they interact with eachother in a way that allows certain wavelengths of our EMR to move through them very well. This is also why when you tap/hit/rub a piece of metal you can hear it, whereas if you were to hit a piece of wood or a rock it wouldn't make as much sound (these objects have molecules that are much further apart and non-uniform so they absorb sound rather than conduct it).

We said that infrared was the portion of the EMR spectrum that corresponded to vibration and thus sound, but remember that a spectrum is continuous. There are individual wavelengths of sound which result in pitch.

Pitch is just the musical term for wavelength. They are one and the same. So when one wavelength or pitch is played near the sheet of metal, the vibrations from the electric keyboard play through a speaker (I won't go into how that works but it's both complicated and cool and has to do with wavelengths and electricity).

The speaker vibrates the air, which travels as a wave with the exact wavelength of the key/pitch chosen and played, and hits the metal. The metal conducts the wave in a certain way through the molecules of metal and vibrate in certain spots.

The spots where the metal plate vibrate hit the grains of sand which makes them jump and move. But certain spots on the metal plate are the spaces between waves (think again of waves at the beach). These "troughs" between the waves act like valleys between mountains. They are dead spots where the metal is not vibrating and so while all of the sand on the vibrating parts of the metal plate are jumping and moving, they will all end up piling into the spaces where they can and will eventually rest.

Change the wavelength, change the pitch, change the pattern of the wave vibrating through the air and then the metal plate, and you will change the pattern of vibration on the plate and thus where the sand jumps/shakes/leaves and where it collects.

Sorry for the long winded answer, I could go into more detail but the nature of an ELI5 means you have to use more words to dumb it down. If I assume you know anything already I won't have to explain it, but what 5 year old knows about frequency and wavelength?

TL:DR Key go baaaaa, metal also go baaaa, molecules in metal shake in shape of baaaaa according to vibration wavelength, sand move to where metal is not vibrating. Key go doooo, metal also go doooo, molecules in metal change to shaking in pattern of doooo, sand moves again to where metal is not vibrating. So on and so forth.