The Mani mantra is the most widely used of all Buddhist mantras, and open to anyone who feels inspired to practice it -- it does not require prior initiation by a lama (meditation master).
The six syllables of the mantra, as it is often pronounced by Tibetans -- Om Mani Padme Hum -- are here written in the Tibetan alphabet:
Reading from left to right the syllables are: Om (ohm) Ma (mah) Ni (nee) Pad (pahd) Me (may) Hum (hum)
The vowel in the sylable Hu (is pronounced as in the English word 'book'. The final consonant in that syllable is often pronounced 'ng' as in 'song' -- Om Mani Padme Hung. There is one further complication: The syllablePad is pronounced Pe (peh) by many Tibetans: Om Mani Peme Hung.
People who learn about the mantra naturally want to know what it means, and often ask for a translation into English or some other Western language. However, Om Mani Padme Hum can not really be translated into a simple phrase or even a few sentences.