"The word mantra has two parts: man, which is the root of the Sanskrit word for mind; and tra, which is the root of the word instrument. A mantra is therefore an instrument of the mind, a powerful sound or vibration that you can use to enter a deep state of meditation.
According to the Vedic tradition, the ancient sages were able to hear the subtle vibrations produced by everything in nature―the sounds of the wind, thunder, butterflies, rushing rivers and all other creations. They recognized that these sounds are the manifestation of spirit into matter. They identified “Om” (or aum) as the most elemental sound, representing the infinite universal consciousness. For thousands of years, people have used this mantra to expand their awareness of the divine.
The ancient seers also identified all the primordial vibrations or mantras that make up the universe and these were eventually recorded in the Vedic literature; The four texts that form the basis for the Hindu religion. You can actually hear all the mantras yourself if you sit quietly. You’ll notice a background hum in the air, and as you practice focusing on that hum, you’ll ultimately hear every mantra the sages recorded long ago."
-What is a Mantra?