In contrast to most other thermodynamic data, entropy values are not given in relation to a certain--more or less arbitrarily defined--zero level. They are listed in standard thermodynamic tables as absolute values of specific substances. Therefore these values describe a physical property of the listed substances. One of the main tasks of chemistry is to understand physical properties with respect to the internal structure of the substances. This comprises issues such as the kind of chemical bonding, particle masses, bond distances, bond forces, molecular volume, density, electronegativity, and so forth. A quantum physical approach on the basis of energy quantization leads to three simple rules, which will be elaborated in this article and which are key to understanding the physical property described by molar entropy values. Substances form a system that is able to store energy if work or heat are added to the system. It is shown that entropy describes the extent of the energy storing system formed by the particles of the substances. The concept of atomic entropy is introduced so that entropy values of substances with different stoichiometry may be compared much more rationally than on the basis of the values of molar entropy. (Contains 20 figures, 1 table, and 6 notes.)