We have reactants A and B that are being consumed and C and D, the products are being formed. Now because we have a one to one to one to 1 mol ratio, because the coefficients are one to one to one to one and because these are being consumed and these are being produced at the same time these are being consumed. These four equations basically describe the rates of these individual reactants. So as the reaction is progressing from the reactant side to the product side, a is being consumed. And that's why the rate of A is negative and the rate of A is simply given by the change in the concentration of A with respect to some time interval and likewise B is also being consumed. So the rate of B is negative and it's given by delta the change in concentration of B with respect to the time and likewise over the same time interval.