Om — The Sacred Syllable Indicating Ishvara
Om (Pranava) is Patanjali's sacred symbol for Ishvara — the vibration that most completely represents the nature of pure consciousness, and the practice of whose repetition and contemplation destroys all obstacles and produces inward-turning consciousness.
Sutra 1.27 identifies Tasya Vachakah Pranavah — 'His (Ishvara's) indicator is Om (Pranava).' Sutra 1.28 prescribes: 'Tajjapas tad artha bhavanam' — the repetition of Om with meditation on its meaning. Sutra 1.29 lists the results: 'Tatah pratyak cetana adhigamo api antaraya abhavash ca' — from this, inward-turned consciousness is gained and all obstacles dissolve. The Mandukya Upanishad identifies the three phonemes of Om (A-U-M) with the three states of consciousness (waking, dreaming, deep sleep) plus the silence between them (Turiya — the fourth, transcendent state). In practical terms, Om is the most direct available gateway to Ishvara Pranidhana through sound: the specific vibratory quality of Om resonates with the frequency of pure awareness in a way that no other sound does, and sustained, devotional repetition of Om with full attention to its meaning is among the most powerful available practices for the stilling of mental fluctuations.
His indicator is the sacred syllable Om. Its repetition with meditation on its meaning is the practice.
Sutras 1.27-1.28 identify the syllable Om (pranava) as the vachaka (expresser) of Ishvara and prescribe its repetition (japa) together with contemplation of its meaning (tad artha bhavanam) as a direct means to inward absorption.
Yoga Sutras of Patanjali