To understand this concept we must be aware of Threshold energy or activation energy.
The threshold energy or the activation energy is the minimum amount of energy that is required for the conversion of reactants into the products .
According to collision theory the reactant species collide with each other to make the product. But every collision don’t bring about the change.
A fraction of collision that bring about the change are called the effective collisions.
Now why we require it ?
According to the collision theory every collision is not an effective collision, to make every collision an effective one ,it must have considerable energy equal to or higher than the threshold energy.
During the chemical reaction the bond breaking takes place first and then new bonds are formed, bond breaking requires energy.