If you remember your high-school physics, free energy is the energy available to do work. Energy is conserved, but free energy is not. For example, when a heavy ball drops from a height, the free energy stays roughly constant (ignoring drag) right until the moment of impact. The amount of free energy lost via inelastic collision is proportional to the height lost at the peak of the bounce. The rest is turned into useless heat. With each bounce, more free energy is lost, until finally all of it is lost. In a world without non-conservative forces like friction (which lower free energy), a ball could bounce for ever. Satellites orbiting earth approximate this: orbital motion is useful work that can be continued indefinitely for free.
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