Mantras in Hindu and Buddhist traditions are not merely aids to concentration; they are understood as sonic forms of the deities or principles they invoke. Om contains the whole of sound; Om Mani Padme Hum embodies Avalokiteshvara’s compassion; the bija (“seed”) mantras of Tantra are each a door.
Related practices across traditions: Sufi Dhikr, the Christian The Jesus Prayer, Jewish shema recitation, Nichiren Buddhist nam-myoho-renge-kyo. Different metaphysics; the same phenomenology of the heart held by a chosen phrase.