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.
All that is real in me is God; all that is real in God is I. The gulf between God and human beings is thus bridged. Thus we find how, by knowing God, we find the kingdom of heaven within us.
-Swami Vivekananda