The rate of reaction can be calculated from the gradient of a curve.
At its steepest, is the highest gradient, therefore the highest rare of reaction as there is a maximum number of reacting particles present and no. of effective collisions per second is at its greatest.
When it is not as steep, it has a lower gradient, therefore a lower rate of reaction as there are fewer reacting particles present therefore number of effective collisions per second is lower.
If gradient is 0, the reaction is complete and at least one of the reactants have been completely used up.