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.
Learn to feel yourself in other bodies, to know that we are all one. Throw all other nonsense to the winds. Spit out your actions, good or bad, and never think of them again. What is done is done. Throw off superstition. Have no weakness even in the face of death. Do not repent, do not brood over past deeds. Be aazaad (free).
-Swami Vivekananda