From HHDilgo Khyentse Rinpopche:
Why is it so important to recite Mantras and what are they? Just as we visualize ourselves as a deity and the surroundings as a buddha-field in order to purify our impure perception of form, we recite mantras to purify our impure perception of sound. Mantra is a Sanskrit word that means “to protect the mind” since, while reciting mantras the mind is protected from its ordinary deluded thoughts. – HH Dilgo Khyentse Rinpoche.