Density is defined as the mass per volume of any given substance.
The plastics have various densities because of their molecular structures, and the solutions have differing densities because of the salt concentrations they contain. Salt water is more dense than pure water because the salt in it contributes to the mass of the entire solution. A solid will only float in a liquid if it is less dense than a liquid. The less dense the plastic is, the higher up the tiered column it goes. Each piece floats on top of the least dense layer that can still support it.