Activation energy is the energy required to start a chemical reaction.
In the diagram on the right, the energy is taken from the surroundings and stored as potential energy in the products. (a) is the activation energy for the reaction to take place whilst (c) is the energy change (known as enthalpy change). (b) is the activation energy needed for the reverse reaction to happen. Here 200 units are turned into Potential Energy and therefore removed from the surroundings, lowering the temperature.