O Satyakama, AUM is the Supreme Brahman and the conditioned Brahman. By meditating upon It the wise man may attain either the one or the other.
If he meditates upon one letter (matra) alone and is enlightened thereby, then after death he will quickly be reborn on this earth. The Rik verses lead him to the world of men, and by practicing austerity, chastity and faith, he will enjoy greatness.
If he meditates on the second letter, he will hold the mind and will be led up by the Yajur verses to the intermediate realm, the plane of the moon. Having enjoyed greatness there, he will return again to this earth.
But if he meditates upon the Highest Being through the word AUM, consisting of three letters, he will be united with the effulgent sun. He will be freed from sin even as a snake is freed from its skin, and he will be led up by the Sama verses to the realm of Brahma. From here, the aggregate of all lives, he will behold the Supreme Purusha, higher than the High and dwelling in the body.
Thus it is written: “The three letters AUM are separately mortal; when joined together in meditation on Reality as a whole and used rightly in the external, internal and intermediate states, the Knower trembles not.”
— Prasna Upanishad