For a chemical reaction to occur, the reactant particles must collide with each other.
However, a collision with too little energy will not produce a reaction. The colliding particles must have enough energy for the collision to be successful or effective in producing a reaction.
The minimum amount of energy for a collision to be successful is called the activation energy.
The rate of a reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction.