The RATE CONSTANT is a constant of proportionality that links the reaction rate to the concentrations of the reagents that actually affect the rate.
The concentration terms in the rate equation may be raised to various indices called ORDERS. The overall order of a rate equation is thus the sum of the individual orders. Remember that the orders HAVE to be determined experimentally.
Thus, if rate =k [X][Y]2 then the rate order with respect to [X] is 1 and the order with respect to [Y] is 2 and the overall order is (1+2) =3.