Simply mixing the reactants do not initiate chemical reactions in most of the cases. It often takes rigorous heating or exposure to other forms of energy to initiate a chemical reaction. The minimum extra amount of energy above the average level that must be supplied to the reactants in order to start a chemical reaction is the activation energy for that reaction. It is conceptually similar to the act of pushing an object to the top of a hill so that it can then slide down to the other side.
Reaction rate is directly tied to activation energy. The lower the energy of activation, the lesser is the energy barrier that must be crossed to go from reactants to products and faster the reaction will take place.
Quantitatively, activation energy is related to reaction rate as: `k=Ae^-(E_a/(RT))`
Where k is the specific reaction rate, `E_a` , the activation energy and A is the frequency factor, characteristic of a particular reaction.