Adding a catalyst can increase the rate of a reaction by providing an alternate pathway for the reaction to occur. The alternate pathway is faster because it has a lower activation energy (AE), so the reactant molecules do not need to collide with as much energy in order for the collision to result in a reaction. Hence, more of the collisions between reactant molecules result in a reaction, so the reaction rate is increased.

Note that a catalyst does not increase the total number of collisions, just the percentage of the collisions that result in a reaction.