Can any one explain the working of the certain part of the given circuit?
R wants to discharge C1 to 0.  Each time the one shot fires the current source get integrated for for the one shot on time.  This raises Vo.  At a fixed frequency the increase in during the one shot on time will equal the decrease during the off time.  The average of this waveform is a function of frequency.  If the frequency is increased, the capacitor will charge the same amount during the one shot on time, but will discharge less because of reduced time between triggers. This will raise the average voltage.