Only effective collision lead to the formation of products. It means that collisions in which molecules collide with sufficient kinetic energy (called threshold energy = activation energy + energy possessed by reacting species). And proper orientation lead to a chemical change because it facilitates the breaking of old bonds between (reactant) molecules and formation of the new ones i.e., in products. e.g., formation of methanol from bromomethane depends upon the orientation of the reactant molecules.
The proper orientation of reactant; molecules leads to bond formation whereas improper orientation makes them simply back and no products are formed. To account for effective collisions, another factor P (probability or steric factor) is introduced