The Om mani padme hum (Derived from the Sanskrit, Devanagari ॐ मणि पद्मे हूँ, IAST oṃ maṇi padme hūṃ) is probably the most famous mantra in Buddhism, 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. It is commonly carved onto rocks or written on paper which is inserted into prayer wheels. When an individual spins the wheel, it is said that the effect is the same as reciting the mantra as many times as it is duplicated within the wheel.
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 then inserted into prayer wheels to increase its effects. Source – Wikipedia