Yōga Sūtra 1.28

तज्जपस्तदर्थभावनम्।

By its repeated utterance (japa) one understands and feels it.

Repetition of mantra is called Japa and is one of the ancient methods employed in meditation. The mantra alluded to here is pranava i.e 'Ōm'. Of all the mantras, 'Ōm' is considered as highest in Indian tradition, as it directly allows the practitioner to understand and feel all-knowing īśvaraḥ, the highest principle. There are mantras with other syllables (e.g 'klīm' for prakruti, the material world) which represent other principles.