What is the activation energy of a reaction?
The energy that must be supplied to make th ereaction happen. It can be thought of as the "energy barrier" for the reaction. If the activation enery is not supplied, the reaction will not happen.
The rate of the reaction will depend on the activation energy. Reactions with large activation energies will generally be slower than reactions with smaller activation energies.
On an energy diagram, the activation energy is the energy difference bewteen the energy of the reactants and the highest point on the energy diagram.