Om mani padme hum

om manipadme hūṃ known as the "six-syllable mantra."
written in Tibetan script on a rock outside the Potala Palace in Tibet.

Om mani padme hum, mani meaning the jewel and padma - the lotus. The six syllabled mantra of the bodhisattva of compassion, Avalokiteshvara (Tibetan Chenrezig, Chinese Guanyin). The mantra is particularly associated with the four-armed Shadakshari form of Avalokiteshvara.

The Dalai Lama is said to be an incarnation of Chenrezig or Avalokiteshvara, so the mantra is especially revered by his devotees and it is commonly carved onto rocks and written on paper which is inserted into prayer wheels, said to increase the mantra's effects.

H.H. the 14th Dalai Lama's definition:

"It is very good to recite the mantra Om mani padme hum, but while you are doing it, you should be thinking on its meaning, for the meaning of the six syllables is great and vast... The first, Om symbolizes the practitioner's impure body, speech, and mind; it also symbolizes the pure exalted body, speech, and mind of a Buddha."

"The path is indicated by the next four syllables. Mani, meaning jewel, symbolizes the factors of method: the altruistic intention to become enlightened, compassion, and love."

"The two syllables, padme, meaning lotus, symbolize wisdom."

"Purity must be achieved by an indivisible unity of method and wisdom, symbolized by the final syllable hum, which indicates indivisibility."

"Thus the six syllables, om mani padme hum, mean that in dependence on the practice of a path which is an indivisible union of method and wisdom, you can transform your impure body, speech, and mind into the pure exalted body, speech, and mind of a Buddha."

-- H.H. Tenzin Gyatso, 14th Dalai Lama, "Om Mani Padme Hum"

http://en.wikipedia.org/wiki/Om_mani_padme_hum

http://www.dharma-haven.org/tibetan/meaning-of-om-mani-padme-hung.htm

http://www.khandro.net/practice_mantra.htm

http://babelstone.blogspot.com/2006/11/mani-stones-in-many-scripts.html