AUM - The PRANAVA mantra

Mundaka Upanishad

Having taken as a bow the great weapon of the Secret Teaching, One should fix in it the arrow sharpened by constant Meditation. Drawing it with a mind filled with That (Brahman) Penetrate, O bright youth, that Immutable Mark. The pranava (AUM) is the bow; the arrow is the self; Brahman is said to be the mark. With heedfulness It is to be penetrated; Become one with It as the arrow in the mark. Know that Self alone, the One without a Second, On which are strung Heaven and Earth, Inner Space, Mind, Vital Energy, all the organs. Leave off other words, for this is the Bridge to Immortality.

Prasna Upanishad

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 practising 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 of 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."

Another source

OM, when considered as one letter, uttered by the help of one articulation, is the symbol of the Supreme Spirit. 'One letter (OM) is the emblem of the Most High, Manu 11,83.' But when considered as a triliteral word consisting of a, u, m, it implies the three Vedas, the three states of human nature, the three divisions of the universe, and the three deities - Brahma, Vishnu and Siva, agents in the creation, preservation and destruction of this world; or, properly speaking, the three principal attributes of the Supreme Being personified in those three deities. In this sense it implies, in fact, the universe controlled by the Supreme Spirit.

Brihadaranyaka Upanishad

AUM.
Lead me from the unreal to the Real.
Lead me from darkness to Light.
Lead me from death to Immortality.