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.
Anything that is secret and mysterious in these systems of yoga should be at once rejected. The best guide in life is strength. In religion, as in all other matters, discard everything that weakens you, have nothing to do with it.
-Swami Vivekananda