Reactions can only occur when reacting particles collide with enough energy.
This energy is called the activation energy
Increasing the temperature of the reaction increases the speed at which particles move. There are more collision and the collisions have a higher energy. This increases the number of effective collisions. Therefore the rate of reaction is increased.
Increasing the concentration of reactants increase the number of particles available to react. More collisions occur. Therefore the rate of reaction is increased.
Decreasing the size of particles increases the surface area. If there is a larger surface available for particles to collide with there will be more collisions. Therefore the rate of reaction is increased.
This video explains about the rate of chemical reactions
Catalysts provide a different route for the reaction to happen. This route has a lower activation energy. This means that more of the collisions will be effective. The rate of reaction is increased.