The activation energy is the minimum amount of the energy which is required to start the chemical reaction.
During a chemical reaction, the reactants are required to gain certain minimum amount of the energy to start the chemical reaction, this minimum amount of the energy is called as activation energy.
The reactant molecules gains the activation energy and called as activated molecules, which gains in kinetic energy and result in effective collision forming the products.
In case of normal chemical reaction the activation energy may be large, but if the same reaction is catalyzed the activation energy required becomes less.
See the image below:
Catalysed reaction has less Ea energy
Non-Catalyzed reaction has more Ea