That is called mantra by the contemplation on which the Individual Soul is released from the Cycle of Reincarnation and attains Final Liberation.
The word mantra was first used in reference to the poetic hymns found in the Rig Veda, the first of the four ancient Vedic scriptures. It is a Sanskrit word that is made up of two distinct words: manas, which means the mind, and tra, which means an instrument of. Combined, the word literally means an instrument of the mind. As such, all mantras share the same purpose: to grow in self-awareness. The ubiquitous mantra in Hinduism is AUM.
If mantra is a sound bite that is used as an instrument of the mind for spiritual growth, then mantra yoga is the practice during which you repeat that sound bite softly to yourself to quiet your mind and direct your attention inward.
Harbhajan Singh Yogi (aka Yogi Bhajan) was a Sikh spiritual leader who introduced Kundalini Yoga to the United States. He asserted that mantra yoga is a science that is based on the knowledge that sound is a form of energy that has structure, power, and a predictable effect on the various energy centers (chakras) throughout your subtle body.
Common mantras used include Hari Om Tat Sat, Om Gam Ganapataye Namaha, Om Hreem Sri Laxmi Bhyo Namaha, Sri Bhagavate Namaha, Om Shreem Hreem Saraswati Namaha, So Hum.