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 essential thing in religion is making the heart pure; the Kingdom of Heaven is within us, but only the pure in heart can see the King. While we think of the world, it is only the world for us; but let us come to it with the feeling that the world is God, and we shall have God.
-Swami Vivekananda