(underlining mine)"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[…]"
I liked that explanation. But HHDL then says that while saying the mantra you should keep in mind the following:
(underlining mine)Thoughts to be had whilst chanting transcribed in simplification keeping essence:
Being and non-beings proliferate loving compassion and indivisible intelligent equanimity; Om Mani Padme Hum.
That is the natural ubiquitous pervasive force of consciousness. These frequencies are in the Sanskrit tongue, act as a harmonic sound resonance against blocking energy, or sleeping energy. Plants reflect this action as well because of the phonetic strength of vibration that is stimulated by natural pronunciation.
—H.H. Tenzin Gyatso, 14th Dalai Lama, "On the meaning of: OM MANI PADME HUM"
The underlined part sounds very Dzogchen-esque to me.
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Then Wiki had a completely different explanation/translation from Dilgo Kyentse, but I liked HHDL's enough to post.
And of course being Wiki, all appropriate caveats are in force.