Repolarization is the return of the membrane potential from the action potential (+35 mV) to the resting potential (-70 mV).

When the membrane reaches its action potential voltage-gated sodium channels close and voltage-gated potassium channels open. So sodium stops entering into the cell and potassium starts to exit. Therefore the repolarization is due to exiting of potassium cations from the cell.

The repolarization causes the potential difference temporarily to increase under –70 mV, below the resting potential, a phenomenon known as hyperpolarization.   _{Your Comment}