The sixth of the eight limbs of yoga is Dharana. Dharana means concentration, but more specifically it means holding the mind steady on a single focus. By focusing on an object in the mind, such as the lotus, a deity, or even the breath (Pranayama), we take the initial step for entering into the deep state of meditation, Dhyana. In practicing Dharana, we are still meditators aware that we are meditating, sustaining the flow of our concentration to purify the mind and our intelligence (buddhi) in order to attain true awareness.
As instructed by Sri Krishna in the Bhagavad Gita:
Those who aspire to the state of yoga should seek the Self in inner solitude through meditation. With body and mind controlled they should constantly practice one-pointedness, free from expectations and attachment to material possessions.
Select a clean spot, neither too high nor too low, and seat yourself firmly on a cloth, a deerskin, and kusha grass. Then, once seated, strive to still your thoughts. Make your mind one-pointed in meditation, and your heart will be purified. Hold your body, head, and neck firmly in a straight line, and keep your eyes from wandering. With all fears dissolved in the peace of the Self and all desires dedicated to Brahman, controlling the mind and fixing it on me, sit in meditation with me as your only goal.
And as Buddha explained in the Anguttara Nikaya:
Thus (referring to attachments) concentration is not yet properly calm or refined; it has not attained full tranquility, nor has it achieved mental unification. But there comes a time when the practitioner’s mind gains firmness within, settles down, becomes unified and concentrated. With such a concentration the practitioner is able to direct the mind to states of higher insight.
True concentration is an unbroken thread of awareness. – B.K.S. Iyengar
In my travels I spent time with a great yogi. Once he said to me,
“Become so still you hear the blood flowing through your veins.”
One night as I sat in quiet, I seemed on the verge of entering a world inside so vast
I know it is the source of all of us.
-Mirabai, 15th century Indian saint