Oṃ Maṇi Padme Hūṃ
This six-syllable mantra is perhaps the most widely known in Tibetan Buddhism, associated with Avalokiteshvara, the bodhisattva of compassion. Each syllable carries profound meaning and is said to purify different negative qualities.
The phrase is often translated as "The jewel is in the lotus," though its meaning extends far beyond literal translation. It represents the union of method (compassion) and wisdom, the inseparability of subject and object, the interpenetration of all phenomena.
Reciting this mantra is believed to invoke the blessings of compassion, to purify negative karma, and to develop bodhicitta - the mind of enlightenment that seeks awakening for the benefit of all beings.
The Dalai Lama has said that the six syllables encapsulate the entirety of Buddhist teaching. Through repetition with proper motivation, practitioners connect with the energy of compassion that pervades all existence.
Source
Tibetan Buddhist Tradition