Reference Electrode-This is the electrode whose potential is constant enough that it can be taken as the reference standard against which the potentials of the other electrodes present in the cell can be measured. Commonly used reference electrodes are the silver-silver chloride electrode (Ag/AgCl/4M KCl, e=0.222 V) or the Calomel electrode (Hg/HgCl/KCl).

In what purpose we use and what is the significance

The answer given by Naveen is correct and all I would add is that the value of the potential is important in order to determine the identities of the various peaks in the CVs. One can very simply think of the magnitude of the peak current as a measure of the amount of the electroactive species (quantitative analysis) and the peak potential as the means of identification of the particular species (qualitative analysis). Of course, there is much more information that can be gleaned from these measurements.

Thanks Dr. Naveen and Dr. Richard A Durst to sharing the valuable suggestions.