The activation energy of a reaction is the minimum energy that must be overcome in order for the reaction to take place. One way of reaching the activation energy is by manipulating the process conditions like pressure or temperature. But the most common method is by adding an enzyme. An enzyme speeds up the rate of the reaction but does not actively take part in it.
An analogy would be pushing heavy wooden block down a slope. No matter how many people push on it, the block won't move because of friction. But if you spill oil on the floor, the block would effortlessly move down the slope. The oil here is like an enzyme in a reaction.
