A mantra is a properly repeated hymn or formula used in rituals and meditation by hindus but also jaïns, buddhists. First uttered by an inspired seer (“rishi”, a prophet who lived something like 4000-3000 years ago, one of the sage who is at the origin of the Vedas), the mantras have been transmitted orally through the centuries from masters to disciples in a carefully controlled manner. It can be composed of a single sound ( “om”, the eternal syllable in hinduism, a sound that encompasses the whole universe, the past, the present, the future) or several words. In hinduism, each deity is represented by its own mantra ( there is an endless number of mantra). It is only by the correct pronunciation of the correct mantra that the deity will enter the body of its image ( then, the image become alive) or will respond to a devotee. So, a mantra is very powerful : it represents the Supreme Being, Brahman, in the guise of sound.
A popular mantra is the “Gayatri mantra” ( Gayatri is the wife of Brahma, the god who created the universe according to hindus). It is considered one of the most revered, one of the oldest too.
“Oh God, you are the giver of life,
Remover of pain and sorrow,
The bestower of happiness,
Oh, Creator of the Universe,
May we receive your supreme sin-destroying light,
May you guide our intellect in the right direction.”
In other words,
“Om bhoor bhuvah swah
Tat savitur varenyam
Bhargo devasya dhimahi
Diyo yo nah prachodayat”
According to the tradition, only the twice born, the nobles ( the Arya), are allowed to recite this hymn. Not so long ago, they were the only ones who knew it, other hindus having no access to the brahminical education at all. It is believed that the stability of the world depend on the rituals performed three times daily by the twice born and the recitation of this mantra. To be effective, it has to be chanted in sanskrit not in any other language. Chanting a mantra with mistakes in it is even worse than chanting no mantra at all.
Some mantras are aimed to attract the opposite sex ( most of the important mantras come from the Tantras, ancient ritual and yogic treatises, esoteric books with mantras containing unexplainable powers). Hindus consider the very name of Jesus is a very powerful mantra for christians, as well as “hail Mary”. Or “Bismillah Ir-Rahman Ir-Rahim” ( In the name of Allah, the merciful, the compassionate) for muslims.