The mantra OM MANI PADME HUM (or HUNG) sometimes gives rise to fanciful or mysterious translations. However, it is simply one name of Chenrezig placed between two sacred and traditional syllables, OM and HUM.

OM represents the body of all Buddhas; it also begins nearly all mantras;
MANI means "jewel" in Sanskrit;
PAD ME, the Sanskrit pronunciation, or PEME in Tibetan means "lotus";
HUM represents the mind of all Buddhas and often ends mantras.
MANI refers to the jewel that Chenrezig holds in his two central hands and PADME to the lotus he holds in his second left hand. Saying OM MANI PADME HUM names Chenrezig through his attributes: "the one who holds the jewel and the lotus." "Chenrezig" or "Jewel Lotus" are two names for the same deity.

Each syllable allows us to close the door of painful rebirths in one of the six realms composing cyclic existence:

OM closes the door of rebirths in the world of the gods (devas)
MA the door of the world of demigods (asuras)
NI the door of the human realm
PAD the door of the animal world
ME the door of the world of hungry ghosts (pretas)
HUM the door of the hell worlds.
Each syllable has a purifying effect:

OM purifies the veils of body
MA purifies the veils of speech
NI purifies the veils of mind
PAD purifies the veils of conflicting emotions
ME purifies the veils of latent conditioning
HUM purifies the veil that covers knowledge.
Each syllable is a prayer in itself:

OM is the prayer addressed to the body of the Buddhas
MA the prayer addressed to the speech of the Buddhas
NI the prayer addressed to the mind of the Buddhas
PAD the prayer addressed to the qualities of the Buddhas
ME the prayer addressed to the activity of the Buddhas
HUM gathers the grace of the body, speech, mind, qualities, and activity of the Buddhas.
The six syllables correspond to the six paramitas, or transcendental perfections:

OM corresponds to generosity
MA to ethics
NI to patience
PAD to diligence
ME to concentration
HUM to wisdom.
The six syllables correspond to the six buddhas reigning over the six buddha families:

OM to Ratnasambhava
MA to Amoghasiddhi
NI to Vajradhara
PAD to Vairocana
ME to Amitabha
HUM to Akshobya.
The colors that correspond to each syllable are:

OM : white
MA : green
NI : yellow
PAD : blue
ME : red
HUM : black.
Lastly, one links each syllable to the six wisdoms:

OM = wisdom of equanimity
MA = wisdom of activity
NI = the wisdom born of itself
PAD = the wisdom of dharmadhatu
ME = discriminating wisdom
HUM = mirror-like wisdom.

Benefits of reciting the Six Syllable Mantra
Extracted from The Daily Enlightenment
By reciting the mantra, the gates leading to rebirth in the six realms of samsara is closed. This powerful mantra's sound and vibration invoke the blessings of all Buddhas to liberate the sufferings of all sentient beings. It removes negative karmas and defilements like greed, anger and ignorance that causes rebirth in the six realms of samsara namely the hell realms, hungry ghosts realms, animal realms, human realms, demi-god realms and god realms. This mantra is so precious and holy that it embodies the Buddha's holy speech. By listening to it with faith and understanding, one is sure to obtain good rebirths after death. If any animal or insect should hear this mantra before dying, it would be reborn to Amitabha's Pure land. While reciting the mantra with mindfulness and a proper understanding , one is ensured of its effectiveness to increase positive merits and the spiritual power of compassion. By dedicating the merits of recitation to all beings and especially our loved ones in times of pain and sickness, all sufferings will dissolve.