Activation energy is the difference between free energy of:

For a substrate to become product, it goes to a transition state (S++) which is a fleeting, momentary, unstable chemical species. It is neither substrate nor product. It has the highest free energy in a reaction coordinate diagram. This creates an energy barrier for substrate to become a product. This energy is known as activation energy. DGcat and DGuncat is the activation energy (see fig). It is free energy difference between substrate and that of the transition state. Even if the substrate to product formation is exergonic, substrate to transition state formation is always endergonic.