Śrīmad-Bhāgavatam 4.8.44, purport

प्राणायामेन त्रिवृता प्राणेन्द्रियमनोमलम् ।
शनैर्व्युदस्याभिध्यायेन्मनसा गुरुणा गुरुम् ॥ ४४ ॥

prāṇāyāmena tri-vṛtā
prāṇendriya-mano-malam
śanair vyudasyābhidhyāyen
manasā guruṇā gurum

Synonyms

prāṇāyāmena — by breathing exercises; tri-vṛtā — by the three recommended ways; prāṇa-indriya — the life air and the senses; manaḥ — mind; malam — impurity; śanaiḥ — gradually; vyudasya — giving up; abhidhyāyet — meditate upon; manasā — by the mind; guruṇā — undisturbed; gurum — the supreme spiritual master, Kṛṣṇa.

Translation

After sitting on your seat, practice the three kinds of breathing exercises, and thus gradually control the life air, the mind and the senses. Completely free yourself from all material contamination, and with great patience begin to meditate on the Supreme Personality of Godhead.

Purport

In this verse the entire yoga system is described in summary, and special stress is given to the breathing exercises for stopping the disturbing mind. The mind, by nature, is always oscillating, for it is very fickle, but the breathing exercise is meant to control it. This process of controlling the mind might have been very possible in those days millions of years ago when Dhruva Mahārāja took to it, but at the present moment the mind has to be fixed directly on the lotus feet of the Lord by the chanting process. By chanting the Hare Kṛṣṇa mantra one immediately concentrates on the sound vibration and thinks of the lotus feet of the Lord, and very quickly one is elevated to the position of samādhi, or trance. If one goes on chanting the holy names of the Lord, which are not different from the Supreme Personality of Godhead, naturally his mind becomes absorbed in thought of the Lord.

It is here recommended to Dhruva Mahārāja that he meditate on the supreme guru, or supreme spiritual master. The supreme spiritual master is Kṛṣṇa, who is therefore known as caitya-guru. This refers to the Supersoul, who is sitting in everyone’s heart. He helps from within as stated in Bhagavad-gītā, and He sends the spiritual master, who helps from without. The spiritual master is the external manifestation of the caitya-guru, or the spiritual master sitting in everyone’s heart.

The process by which we give up our thoughts of material things is called pratyāhāra, which entails being freed from all material thoughts and engagements. The word abhidhyāyet, which is used in this verse, indicates that unless one’s mind is fixed, one cannot meditate. The conclusion, therefore, is that meditation means thinking of the Lord within. Whether one comes to that stage by the aṣṭāṅga-yoga system or by the method recommended in the śāstras especially for this present age — to constantly chant the holy name of the Lord — the goal is to meditate on the Supreme Personality of Godhead.