This mantra is an aspect of the Dharmakaya, the formless aspect of buddha mind. Sometimes the A OM HUNG is not recited, leaving only the final nine syllables. The meaning of each of the nine syllables is as follows:

AH represents the primordial Buddha, the changeless Essence, the Void.
A The second short A represents the ceaseless clarity.
KAR means the purification of negative emotions, the two obscurations, and all karmas.
SA-LE means beyond all limitations of words and concepts.
OD literally means light, but its significance here means beyond conceptual obscurations and grasping mind.
A represents buddha mind, the omniconsciousness that perceives all instantly.
YANG normally represents the air element, but in this case it is associated with the wisdom prana—that which activates wisdom within oneself and removes obscurations.
OM represents the Five Buddha Families, the Five Wisdoms, and the Five Embodiments.
DU essentializes everything into oneness, the tigle nagchig, or single sphere.

Though the meaning can be broken down syllable by syllable, one should understand that the ultimate meaning of the mantra is the essence of one's own mind.

The particular benefit of the mantra is to remove obstacles of meditation and develop clarity of the view. It is known as the mantra of the dharmakaya or Samantabhadra, or the mantra that develops the view. It is the essence of the actual deity Samantabhadra, so as you recite it you become more closely identified with this deity and all of his perfected qualities.

When you recite the mantra, visualize that you transform yourself into Samantabhadra, Kuzang Gyalwa Dupa, or Shenla Odkar, who are all manifestations of pure buddha mind. You should visualize that you are the umze, or chant master, leading all sentient beings in reciting this mantra, and in this way benefiting not only yourself, but all beings.