Thermodynamic free energy is a state function that measures the maximum amount of useful work obtainable from a closed system, relating the changes in enthalpy and entropy under conditions of constant temperature and pressure. This concept is crucial for determining the spontaneity of physical and chemical processes. The change in free energy ($\Delta G$) dictates whether a reaction will proceed naturally; if $\Delta G$ is negative, the process is spontaneous, meaning the system tends toward a more stable, lower-energy state. Conversely, a positive $\Delta G$ indicates that the process is non-spontaneous and requires an external input of energy to occur. Therefore, understanding free energy allows for the quantitative prediction of reaction feasibility and the equilibrium point of a system.