21 is often the number of chants or mantras used when chanting 'OM' or 'AUM'
What is a mantra? Originally from Hindu and Buddhist teachings, mantra refers to a word, sound, or phrase that is repeated and used to support concentration and/or a meditation practice. It facilitates emptying the mind of everything except the mantra and then after chanting 21 or 108 times, the stillness and peace at the end is so very tangible.
AUM or OM is one of the most powerful mantras of all time.
Words and sounds are basically vibrations. Nikola Tesla said, "If you want to find the secrets of the universe, think in terms of energy, frequency and vibration.” The very foundations of our Universe, of matter and thought, appear to lie in sound vibration."
Patanjali taught: “Chant OM and you will attain your goal. If nothing else works, just chant OM.”
OM, or AUM, is a Vedic Sanskrit word with three sounds/syllables: "A", "U", and "M", (In Sanskrit, the vowels 'A' and 'U' become 'O'). OM is a seed Mantra - a single syllable mantra from the Sanskrit language.
David Frawley, in his book Mantra Yoga and Primal Sound, wrote: Om is the Word of God.” The sound OM is a vibration from which all the manifest universe emanates. Form and creation come from vibration. OM is the most elemental of vibrations. It is the sound of the void. Frawley says: “Om is the prime mantra of the Higher Self or Atman. It attunes us with our true nature. It is the sound of the creator, preserver, and destroyer of the universe, who is also the inner guru and prime teacher. It reflects both the manifest and un-manifest Brahman, sustaining the vibration of being, life, and consciousness in all worlds and all creatures.”
Om represents the whole world and all of its sounds.