The functionality of materials is determined by the properties of and the interactions between its molecules. In practice it is impossible to keep track of all these interactions because of the large number of molecules  in the material. Therefore, one describes the material properties in terms of macroscopic quantities as an average over large numbers of molecules.
In the course Thermodynamics the material properties are described in terms of temperature, pressure and volume, finally leading to the energy of the systems in terms of heat and work. It turns out that even without knowledge of the microscopic interactions between the molecules surprisingly many macroscopic thermodynamic properties can be understood and even forecast.
Two key notions are needed for that: Energy and entropy, which together form the free energy. Free energy is central in thermodynamics as it drives all kinds of processes like chemical reactions, phase transitions and osmosis.