There are various opinions regarding the effectiveness of mantras. Some discount their value altogether, while others practice mantras exclusively and even go to the extreme of completely dismissing traditional meditation practice.
From my own experience, I can say that the practice of mantras can be very powerful. I do not consider it a complete practice. I think the benefits derived from silent meditation are just too great and too many to do without it. Mantra practice is generally consigned to Vajrayana and even most teachers in that tradition consider silent meditation to be a higher practice.
Essentially “mantra” means “to protect the mind.” From what? Delusions, attachments, desire, etc.
Mantra words are not really words at all, they are bija or seed syllables. In most cases, they have no literal meaning, rather they are symbolic, representing a “diety” or a concept, or energy force. For instance, the most famous bija, OM, is the seed syllable of the universe, representing infinite power. AH is associated with karma and Amoghasiddhi, a celestial Buddha. Another bija, HUM, represents truth. OM AH HUM is a well-known mantra.
Technically, devotional incantations to Buddhas, like “Namo Omito-Fo” (Praise to Amitabha Buddha), or to sutras, such as “Namu-Myoho-Renge-Kyo” (Praise to the Lotus Sutra), are not mantras. They may be used in the same way as a mantra, but they are not considered to be as effective.
When we talk about the power of mantra, we mean the power of words, speech, sound. The theory of how mantras work is too complex to go into here. For now, this short explanation by Roger Corless, from The Vision of Buddhism will suffice:
Mantra is dharma manifested as, embodied or incarnated in sound. A mantra may contain words, or sounds that has a specific meaning; but meaning is not its essential feature. A mantra communicates dharma directly to the mind without the meditation of concepts.
Most people when they hear or learn a mantra want to know what it means, and they are disappointed and frustrated to discover that it may have no literally meaning, or that the meaning cannot be told. This is not because they are spells, incantations, or mumbo jumbo, or because they are “secret,” as Lama Govinda discusses in The Foundations of Tibetan Mysticism:
Mantras do not act on account of their own ‘magic’ nature, but only through the mind that experiences them. They do not posses any power of their own; they are only the means for concentrating already existing forces – just as a magnifying glass, though it does not contain any heat of its own, is able to concentrate the rays of the sun and to transform their mild warmth into incandescent heat . . .
Their ‘secret’ is not something that is hidden intentionally, but something that has to be acquired by self-disciple, concentration, inner experience, and insight.
Even though at times it may appear that mantras are outer directed, their function is really to connect us with our inner capacities for compassion, healing, goodness, and wisdom. Another way to put is that they help us tap into our Buddha Nature. Or they help us experience emptiness.
For mantra practice to be most effective, it’s important to maintain single-pointed concentration. Just as in silent meditation, if the mind is wavering, wandering, or otherwise not fully engaged in the present moment, the benefits derived from this sort of practice are greatly reduced.
om sunyata jnana vajra svabhavako’ham