Om Mani Padme Hum is the mantra of Chenrezig. In the words of Kalu Rinpoche, "Through mantra, we no longer cling to the reality of the speech and sound encountered in life, but experience it as essentially empty. Then confusion of the speech aspect of our being is transformed into enlightened awareness."
That enlightened awareness includes whatever we might need to understand in order to save any beings, including ourselves, from suffering. For that reason the entire Dharma, the entire truth about the nature of suffering and the many ways of removing it's causes, is said to be contained in these six syllables. from Dharma-haven.org