For a collision to be effective, the colloiding molecules must have energy more than a particular value known as threshold energy. At room temperature most of the reactant molecules have energy less than the threshold value. When temperature is increased, the reactant molecules absorb this energy and their energy becomes equal to or greater than threshold value. This extra amount of energy absorbed by the reacting molecules is known as activation energy.
Activation energy $=$ Threshold energy $-$ Average kinetic energy of the reactants.
Evidently, the rate of reaction increases on rise in temperature, because average kinetic energy of the reactants increases and activation energy decreases. Also, on rise in temperature, the number of molecules having kinetic energy greater than threshold value increases. Thus, the increase in the rate of reaction with increase in temperature in mainly due to increase in number of effective collisions.