A consecutive reaction $A\stackrel{k_1}{\to }B\stackrel{k_2}{\to }C$ is characterized by

A consecutive reaction is characterized by maxima in the concentration of B. Initially, only A is present. 

As time progresses, more and more B is formed. Hence, the concentration of B increases. But with an increase in the concentration of B, the rate of conversion of B to C also increases. Hence, a maxima is achieved in the concentration of B.