Short answer: Gibbs free energy $G = U + PV - TS$ combines internal energy $U$, pressure $P$, volume $V$, temperature $T$, and entropy $S$ into a single quantity that measures spontaneity. With that, I mean that processes that lower the Gibbs free energy of your system will spontaneously occur, and equilibrium is reached when the Gibbs free energy reaches the lowest possible value.
Processes that increase the Gibbs free energy can be shown to increase the entropy of the system plus it's surroundings, and will therefore be prevented by the second law of thermodynamics. That's why it measures useful energy: your system may contain more energy, but entropy considerations will prevent you from spending it.