O

Om mani padme hum 

The six syllables in Tibet mean ‘Hail to the Jewel in the Lotus’, being the most common mantra (‘protection of the mind’) carved on mani stones. 

In more detail:

Om: symbolize the impure body, speech & mind

Hum: symbolize the indivisibility of method & wisdom

So the full meaning is in dependence on the practice of a path, an indivisible union of method and wisdom, one can transform his body, speech & mind into the pure body, speech & mind of a Buddha. (As we all have within us the seed of purity, we should not seek for Buddhahood outside ourselves.)

Sometimes the first syllable, Om, will be carved (for example on yak or sheep skull) to represent the six syllables.