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.
The moment you fear, you are nobody. It is fear that is the great cause of misery in the world. It is fear that is the greatest of all superstitions. It is the fear that is the cause of our woes, and it is fearlessness that brings heaven in a moment.
-Swami Vivekananda