OM MANI PADME HUM
Om Mani Padme Hum mantra is one of the most popular Buddhist mantras recited by the Tibetans and the Buddhists along the Himalayan belts. What is the inspiration behind this mantra and what does this mantra really signifies. It is Avalokestisvara, the Buddha of compassion, mantra to help rescue all sentient beings, including oneself from the sea of Samsara. This is a general belief and understanding.
Avaloketisvara, revered in Tibet as Chenrezig [Tib: sPyan ras gzigs], is supposed to be the patron deity of Tibet. Manikabum, an ancient text attributed to the scholarship of the 7th century King Srongtsan Gampo, has the whole myth and history about the sacred relationship.
There may be various approaches to explain the meaning of the mantra at different levels. According the Manikabum text, it says that the recitation of the mantra helps one not to be reborn in the six realms [Tib: 'Dro ba rigs drug]. It also helps mitigate the sufferings of the beings in the six realms.
Recitation of the mantra in deep contemplation with a strong motivation to relieve the sufferings of all beings is a good spiritual practice. But in our day to day life, we can recite this mantra even while taking a walk. This will help the health of our body, speech, and mind.
I remember my mother (late) telling me one should recite Mani wherever possible. She said the recitation of the mantra will calm your mind and make you mindful. She has a beautiful Tibetan lyrical stanza, which translate to something like this. Recite Mani even when you are walking, no need to worry that you will trample it; recite Mani even when you are eating, no need to worry that you will swallow it; recite Mani even while in toilet, no need to worry that it will get dirty; recite Mani even when you are sleeping, no need to worry that it will rob your sleep; and so on.
Buddhists usually accumulate the recitations with a rosary of 108 beads, one circle of rosary recitation means Trengkor chig [Tib:Phreng skor gcig].
In one of His Holiness the Dalai Lama's teachings, a young girl asked His Holiness the meaning of Om Mani Padme Hum. Here is the gist of what His Holiness the Dalai Lama said:
Buddhist system tends to utilize the human intelligence in a maximum way to train and transform our mind to achieve enlightenment. Recitation of mantra is one way to achieve transformation of mind.
This Avalokisteshvara mantra, Om Mani Padme Hum has six syllables. Om usually comes first in the Hindu and Buddhist mantras.
Om has three letters, A, O, and Ma. These three letters represent body, speech and mind at two levels. The impure level is the basis of all suffering, and the pure level is the basis of enlightenment.
We practice to transform our body, speech, and mind which are at the impure level to ever pleasant pure level. Now, how do we do this? Mani means Jewel [Norbu], it represents infinite altruism and compassion. Padme means lotus representing wisdom. Hum represents the realization of combination of the altruism and wisdom.
So, Om Mani Padme Hum can be translated as a mantra to transform the impure body, speech, and mind of ours to a pure level through the practice of altruism and wisdom, [thabs dang shes-rab].
This altruism is a Boddhicitta mind to help all sentient beings, and the wisdom is the wisdom realizing the emptiness of all phenomena.
But mere recitation of prayers and mantras is not helpful; it should be complimented with a real practice. All religions carry same message of love, compassion, and tolerance. Theistic religions lay more emphasis on prayers. Buddhism is non-theistic, it emphasis is more on practice than prayers.
Buddha has said, you are your own master and savior. Your future directly depends on yourself. Study and accumulation of knowledge is very important. But to use this knowledge in the best possible way, it is important to have a good motivation, i.e. Bodhicitta mind. Therefore, having a good and kind heart is very important.
One of the famous quote of His Holiness the Dalai Lama is "Kindness is my religion", and "Help other, if you cannot help, at least don't harm them".
Here is a beautiful recitation of the mantra in musical form: https://www.youtube.com/watch?v=R-ZO7bsA2pA
 Manikabum scripture, p-36, Shering Parkhang. ཨོྂ་གྱིས་ལྷའི་སྐྱེ་སྒོ་གཅོད། མས་ལྷ་མ་ཡིན་གྱི་སྐྱེ་སྒོ་གཅོད། །ཎིས་མིའི་སྐྱེ་སྒོ་གཅོད། མེས་ཡི་དྭགས་ཀྱི་སྐྱེ་སྒོ་གཅོད། ཧཱུྂ་གིས་དམྱལ་བའི་སྐྱེ་སྒོ་བཅད་ནས། འགྲོ་བ་རིགས་དྲུག་གི་གནས་སྟོངས་པར་བྱེད་པ་ཡིན།
 The six realms are: three upper realms of gods, demi-gods, and human, and three lower realms of hell beings, petras, and animals
 I am trying to get this original Tibetan stanza, if anybody knows it, please share at the comment box.
 Here is the video link: https://www.youtube.com/watch?v=4QQKDP8SQZg
 བདག་ཉིད་བདག་གི་མགོན་ཡིན་གྱི། གཞན་ལྟ་སུ་ཞིག་མགོན་དུ་གྱུར།