In chemistry, the spontaneity criterion for a basic chemical reaction, at constant temperature and pressure, is that the calculated measurement of the Gibbs free energy change ΔG for the reaction process, over the extent of the reaction, is a negative value. [1] In translation, for any generic reaction between reactants and products:

Reactants → Products

such as in a combination reaction:

A + B → AB

the "spontaneity" criterion equates to the following set of three rules:

ΔG < 0 – the reaction is spontaneous in the forward reaction
ΔG > 0 – the reaction is nonspontaneous (the reaction is favored in the opposite direction)
ΔG = 0 – The system is at equilibrium (there is no net change)

In this sense, the Gibbs free energy G or simply “free energy” represents, in the words of American chemist Raymond Chang, “the energy available to do work”. [2] Here, G equates to instanteous measurements of:

G = H – TS

of each component species, A, B, AB, etc., per unit time, where H is the enthalpy component, T is the absolute temperature of the system, and S is the entropy component. Thus, if a particular reaction is accompanied by the release of usable energy (i.e. if ΔG is negative), this fact alone guarantees that it will be spontaneous and will go naturally on its own without an external energy input.

References
1. Chang, Raymond. (1998). Chemistry, 6th ed. (ch. 18: “Entropy, Free Energy, and Equilibrium”, pgs. 725-55). New York: McGraw-Hill.
2. ibid (pg. 738).