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.
Bless people when they revile you. Think how much good they are doing by helping to stamp out the false ego. Hold fast to the real Self. Think only pure thoughts, and you will accomplish more than a regiment of mere preachers. Out of purity and silence comes the word of power.
-Swami Vivekananda