A sling shot metaphor can be used to help understand activation energy. In endothermic reactions—like the force used pull the rubber strip of a sling shot—activation energy is the energy invested to raise reactant molecules to an "elevated" intermediate state. In exothermic reactions—like the force of letting go of the rubber—activation energy is also the energy required to release the reactant molecules in exothermic reactions. Enzyme catalysis can reduce the amount of activation energy needed.
Change in Free Energy
Change in free energy is the amount of energy that occurs at a standard condition in which there is a precise temperature, pressure and concentration of reactants and their products. It is calculated as the difference between free energy of the reactants and the content of the products in that standard condition.
Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism, 5th ed. Belmont, CA: Thomson Wadsworth, 2009, pp 22-26.