Consider a hypothetical reaction:
A ↔ B
The change in free energy, ΔG can be related to the equilibrium constant K as:
ΔG = -RT ln K
i.e. K = exp (-ΔG/RT)
When ΔG is negative, the exponential becomes positive which results in a high value of the equilibrium constant (K). More and more of the reactants get converted into products and the reaction tends to be spontaneous.