This Upanishad belongs to the Atharva Veda. As the name of the Upanishad suggests, the teachings revealed in it are such as to leave its students clean, like a shaven head (mundaka), rid of the veil of ignorance. The lucid translation of the text along with Shankaracharya's commentary on it is an additional aid to comprehend the profundity of the subject.
Those who have succeeded in attaching or detaching their minds at will have succeeded in Pratyahara, which means gathering towards, checking the outgoing powers of the mind, freeing it from the thralldom of the senses. When we can do this, we shall really possess character; then alone we shall have taken a long step towards freedom. Before that, we are mere machines.
-Swami Vivekananda