Reactions can not occur without an effective collision, The more effective collisions the faster the reaction rate.
If the molecules hit at the wrong angle or without enough force no reaction will take place. An effective collision is one that cause the molecules to break apart so that the molecules can recombine in new combinations.
If the number of effective collisions increases because of increased temperature or increased concentration the reaction rate will increase.
If the number of effective collision decreases because of less kinetic energy or fewer molecules that the reaction rate will decrease.