They are basically tall thin glass jars filled with liquid and a special kind of colored wax, set on top of a base with a light bulb. When the bulb is turned on, the lamp glows, the liquid heats up, and the wax begins to melt. Shake salt on top of the oil while you count slowly to 5. Add more salt to keep the action going for as long as you want. They did it over and over again and kept trying to touch the layers.
Oil floats on water because a drop of oil is lighter than a drop of water the same size.
Another way of saying this is to say that water is denser than oil. Density is a measurement of how much a given volume of something weighs. Things that are less dense than water will float in water. Things that are more dense than water will sink. Even though oil and water are both liquids, they are what chemists call immiscible liquids.