"Om ah Hum" is a well-known Buddhist mantra, but what does it mean? It is a mantra that functions on many levels. The following is a basic explanation.

The Tibetan Buddhist Chant of Om ah Hum also contains these sounds. The Om ah Hum, the Mantra of Blessing frequently precedes the recitation of other mantras.
The sounds Om, Ah and Hum are individual Sanskrit seed syllables. The first thing we notice is that consists of three words, what immediately implies the symbolism of any trinity. In Buddhism the most common found trinity is that of Body (kaya), Sound or Speech (vak) and Spirit or Mind (citta). The perfect state of being of each of these three bodies is symbolized by the three words of Om a Hum. The Body is not only the physical body of man but also the entire material dimension of the individual. The Sound is the life energy of the Body, known as prana. The Spirit is the reason and that what is behind the reason.

Looking a little closer to those three words, we can say that Om is the primal vibration out of which all things came forth and into which all things will be absorbed at the end of the cosmic cycle. Om is the first manifestation from Shiva-Shakti in the form of sound consciousness. At the same time Om is the quintessence of the universe. It is the highest expression of consciousness. Ah is the symbol of the primal state of the spirit. It is the female aspect, the mother fully expressed in divine wisdom. It is also the unborn, a state of being that is without thought. It is the essential state of Emptiness. Hum is the root vibration, the smallest, undividable unity of sound as reflection of the essential nature of Kundalini-Shakti. Hum is the descent from universality (Om) into the human heart.
Macrocosm is reflected in microcosm, and the rather esoteric explanation of Om a Hum is reflected in the chakras. Om is related to the Crown chakra (Sahasrara), and is of the color white (purity). Ah is related to the Throat chakra (Visshudha), and is of the color red (Divine Love).Hum is related to the heart chakra (Anahata), and is of the color blue (stability and eternity).
When pronouncing or chanting Om ah Hum one visualizes a flow between the words and what the words stand for. When pronouncing Om one imagines that it stands for the universal sound, the symbolic word for the infinite, the perfect, the eternal. It fills the mind with the idea of eternal perfection. Om is like the opening of our arms to embrace all that lives and exists. It brings peace, bliss, clarity, firmness, courage, stability and strength. Om descends from the Crown chakra to the heart chakra in order to be transformed into vibrant life. Ah is the expression of wonder and direct awareness. It is a point of stillness, of emptiness. Ah brings energy, openness, expansion and empowerment. Hum is the universality (Om) brought into the heart, it is the infinite in the finite, the eternal in the temporal. The long 'u' sound in Hum expresses a downward movement to accentuate this descent. The 'h' in Hum is the sound of the breath (prana) and 'm' hints at a state beyond duality. Hum is brings expansiveness, infinity, essence and oneness into the human being.
Thus this mantra brings the primal, universal, infinite vibration into one's heart and being, into one's awareness.