Yoga Sutras 1.28
Patanjali, explained by Swami Vivekananda
तज्जपस्तदर्थभावनम् ॥
tajjapastadarthabhāvanam
The repetition of this (Om) and meditating on its meaning (is the way).
In plain words: The practice is to repeat Om and think about its meaning. The teacher explains that repeating it builds good impressions in the mind that awaken holy thoughts.
Swami Vivekananda explains
Why should there be repetition? We have not forgotten the theory of Samskaras, that the sum-total of impressions lives in the mind. They become more and more latent but remain there, and as soon as they get the right stimulus, they come out. Molecular vibration never ceases. When this universe is destroyed, all the massive vibrations disappear; the sun, moon, stars, and earth, melt down; but the vibrations remain in the atoms. Each atom performs the same function as the big worlds do. So even when the vibrations of the Chitta subside, its molecular vibrations go on, and when they get the impulse, come out again. We can now understand what is meant by repetition. It is the greatest stimulus that can be given to the spiritual Samskaras. "One moment of company with the holy makes a ship to cross this ocean of life." Such is the power of association. So this repetition of Om, and thinking of its meaning, is keeping good company in your own mind. Study, and then meditate on what you have studied. Thus light will come to you, the Self will become manifest.
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But one must think of Om, and of its meaning too. Avoid evil company, because the scars of old wounds are in you, and evil company is just the thing that is necessary to call them out. In the same way we are told that good company will call out the good impressions that are in us, but which have become latent. There is nothing holier in the world than to keep good company, because the good impressions will then tend to come to the surface.