Jadis' Secret Police, also known as the White Witch's Secret Police, was the undercover police force established by the ruler Jadis. Known members of the Secret Police include its captain, Maugrim, and some of the ever-listening trees and various other creatures who worked as spies.
This police force was created for the purposes of intimidation, reconnaissance, and spying on the oppressed Narnian citizens. They arrested anyone whom they viewed as traitors or those they were ordered to by Jadis.
With their captain, Maugrim, dead at the hands of Peter Pevensie, the Secret Police was either disbanded right then and there, or at latest, by the time Jadis died at the battle field.