Now, we begin with two reactants, x and Y and they convert into W and Z. What elementary simply means is that they convert from reactants to products in a single step. Elementary reactions are some of the most basic reactions out there. Now notice that A-B-C and D are coefficients of each respective molecule and they represent the number of molecules or moles. Now, rates of reactions tell you how quickly reactants become products, so how quickly x and y become W and Z. In other words, the rate can be found by the change in concentration of reactants over some given time.