The rate of a chemical reaction is directly proportional to the concentration of the reactants raised to a particular exponential power. The value of the exponent is referred to as the order of the reaction with respect to a particular reactant. When the concentration of a particular reactant is changed, the rate of the reaction will change according to the exponent. So if you change the reactant by a factor of 1.5 and the rate changes by a factor of 2.3, the exponent is not 1 because these changes are not the same. If you square the number 1.5 you get 2.25 which rounds to 2.3. So the exponent for this particular reactant in the rate equation is 2. This makes the reaction second order with respect to this reactant.