From the text "Healing Power of Mantra: The Wisdom of Tibetan Healing Science":


The Role of Mantras in Healing
What is a mantra?
A mantra is a set of powerful words that carries certain phonetic vibrations and energy, which when chanted or recited has the power of not only dispelling the disease but also had the power of connecting oneself to higher levels of consciousness. It is a key which unlocks the mysteries of the universe, manifesting and channeling divine energy from higher dimensions. It is a tool, which quickly, swiftly and smoothly deepens your meditative state and insight, if practiced under the guidance of a spiritual master.
A Mantra is a combination of sacred syllables which generate spiritual energy. A Mantra is not a prayer; it is a sacred positive affirmation, recited as a chant. In Sanskrit, the word ‘Man’ means to ‘to think’, while the word ‘tra’ comes from ‘trai’ which means to free oneself from the physical world. The combination of these two words gives ‘Mantra’, which means ‘the thought that liberates and protects’.
Mantra is known by the word ‘sngags’ in the Tibetan language. The holy and sacred Mantras express the fundamental sounds and energies essential for bringing harmony to the elements between body and mind. The chanting of a Mantra brings tremendous healing energy and helps one to achieve balance in body, mind and spirit. Like food for the body, Mantras nourish the individual soul.
In otherwords, mantra is a particular combination of sounds symbolizing and communicating the nature of a deity and which lead to purification and realization, for example ‘om mani padme hung‘. There are three major types of mantra: guhya mantra, vidya mantra and dharani mantra.
Mantras are very powerful healing aids. They are not simply sounds in the conventional sense, but are the resonance of subtle primordial energies that we have within ourselves, the vibrations of which distribute gentle healing energies throughout our being.
Generally, the chanting of Mantra should first be performed loud enough that one can listen to the sound. The vibration of Mantra penetrates deeper and deeper into the heart and finally one may remain silent, working internally with the supersonic sound. It is also important to know that there also some mantras that should chant silently within oneself.
However, there are some Mantras which should not be recited without first being initiated by a Spiritual Master, one who has received authoritative instructions on how to use them, thus allowing him to pass them on to his disciples. Once the Spiritual Master initiates his disciples in the chanting of the Mantra and providing his disciples continue to do so sincerely and as directed by the Master, the power of Mantra grows exponentially.
In Buddhist meditation, many things can be used as objects of concentration. The breath is used in the mindfulness of breathing, the sensations of walking are used in walking meditation, the emotions are used as a focus in metta bhavana (development of loving kindness), and visual images and objects are used in visualization. Mantras are sounds, words or phrases that are also used as an object of concentration. The sounds of Mantra may be chanted out loud, or may be heard internally. Mantras can be associated with particular historical or archetypal figures, or may have no such associations. For instance, there are Mantras associated with the historical Buddha (Om muni muni maha munaye svaha), and Avalokiteshvara (Om mani padme hum). The Prajnaparamita Mantra (Gate gate paragate parasamgate bodhi svaha) is associated not with an enlightened figure, but with a body of texts known as the Perfection of Wisdom (Prajnaparamita) sutras.