यदक्षरं वेदविदो वदन्ति विशन्ति यद्यतयो वीतरागाः ।
यदिच्छन्तो ब्रह्मचर्यं चरन्ति तत्ते पदं संग्रहेण प्रवक्ष्ये ॥
What the knowers of the Vedas declare as imperishable, what the ascetics devoid of passion enter, desiring which they lead a divine life of celibacy, that goal I will describe to you.
The knowers of Veda declare the goal as aksaram. aksaram means that which does not perish or undergo any destruction. This askaram is formless, attribute less - neither gross nor subtle. Therefore nirguna Brahman is meant here. Who says so, the वेदविदो, the knowers of the Vedas. Here knowers means not just one reading the Vedas, but one who dedicate their life for this pursuit. That is why the Lord uses the word यद्यतयो, they strive all life for this goal, all actions are meant for this goal.
That the knows of Veda speak about it finds its reference in the Brihadaranyaka Upanisad. "O Gargi, the knowers of Brahman say, this immutable (Brahman) is that. It is neither gross nor minute, neither short nor long." (Br. 3.8.8)
Having known this aksaram they enter, become one with it. Who? The self controlled sanyaasis.
Bhagavan is giving three practices or sadhana which they practiced before they could become one with this aksaram goal. The practices are the Lord says in this Sloka
(1) यद्यतयो = Continuous life time practice as well as regular.
(2) वीतरागाः = Freedom from attachment, means they practiced sama , dama before. When it is said freed from raga, its opposite dvesha (free from repulsion) must also be taken.
(3) ब्रह्मचर्यं = ब्रह्मचर्यं means celibacy. But it includes in its gambit all scriptural practices like Guru Seva, sravana, mañana and samadhi. For all these actions are only for the knowledge of the Self.
After having practiced these methods they विशन्ति, enter i.e they identify with the aksaram Purusha after disassociating with the BMI.
The पदं, which they seek is Om. Bhagavan Sankara says this practice of OM is prescribed for the madhya (middle) category seekers. As Om involves support of the name, it is for people whose mind has not been made that subtle but at the same time is neither very gross.
Bhagavan has already explained in Sloka 9 the method of jnana. For people who a less evolved and need an additional support (alambana) for their meditation, this Pranava upasana is prescribed.
That one who mediates on this AUM, attains the Purusha is said also in the Prasna Upanisad as follows
"Again, anyone who meditates on the supreme Purusha with the help of this very syllable Om as possessed of three letters, he attains Him (Prasna 5.5).