Verse 8.5.3

अथ यदनाशकायनमित्याचक्षते ब्रह्मचर्यमेव तदेष ह्यात्मा न नश्यति यं ब्रह्मचर्येणानुविन्दतेऽथ यदरण्यायनमित्याचक्षते ब्रह्मचर्यमेव तदरश्च ह वै ण्यश्चार्णवौ ब्रह्मलोके तृतीयस्यामितो दिवि तदैरं मदीयं सरस्तदश्वत्थः सोमसवनस्तदपराजिता पूर्ब्रह्मणः प्रभुविमितं हिरण्मयम् ॥ ८.५.३ ॥

atha yadanāśakāyanamityācakṣate brahmacaryameva tadeṣa hyātmā na naśyati yaṃ brahmacaryeṇānuvindate'tha yadaraṇyāyanamityācakṣate brahmacaryameva tadaraśca ha vai ṇyaścārṇavau brahmaloke tṛtīyasyāmito divi tadairaṃ madīyaṃ sarastadaśvatthaḥ somasavanastadaparājitā pūrbrahmaṇaḥ prabhuvimitaṃ hiraṇmayam || 8.5.3 ||

3. Then, that which is known as ‘anāśakāyana’ [the path of fasting] is brahmacarya, for through brahmacarya one attains the Self which is immortal. Then, that which is called ‘araṇyāyana’ [life in the forest] is brahmacarya. This is because in Brahmaloka, which is the third world from the earth, there are two oceans called Ara and Ṇya. There also one finds a lake called Airammadīya [so-called because its waters are intoxicating], a peepal tree always exuding soma juice, a city called Aparājitā [the Invincible] belonging to Brahmā, and a canopy of gold specially made by the Lord.

Word-for-word explanation:

Atha, next; yat anāśakāyanam iti ācakṣate, that which is called ‘anāśakāyana’ [i.e., anāśaka + ayana, the path of fasting]; tat brahmacaryam eva, that is brahmacarya; hi, for; eṣaḥ ātmā, this Self; yam brahmacaryeṇa anuvindate, which one attains through brahmacarya; na naśyati, never perishes; atha, then; yat araṇyāyanam iti ācakṣate, that which is called ‘araṇyāyana’ [i.e., araṇya + ayana, life in the forest]; tat brahmacaryam eva, that is brahmacarya; tat, there; araḥ ca ha vai ṇyaḥ ca, Ara and Ṇya; arṇavau, are the two oceans; brahmaloke, in Brahmaloka; tṛtīyasyām itaḥ divi, in the third heaven from here [i.e., from the earth]; tat, [and] there; airammadīyam saraḥ, is a lake called ‘Airammadīya’; tat aśvatthaḥ, [and] there is a peepal tree; somasavanaḥ, which exudes soma juice; tat aparājitā pūḥ, [and] there is the city Aparājitā [the ‘Invincible’]; brahmaṇaḥ, belonging to Brahmā; prabhu-vimitam, built specially by the Lord; hiraṇmayam, [a canopy] made of gold.

Commentary:

When you practise brahmacarya, you are always moving on the plane of the Self. Fasting (anāśakāyana) is also called brahmacarya, because when you fast you attain (ayana) the Self that is protected (anāśaka).

So also, if you retire to the forest, that is brahmacarya. You say, ‘I’ve had enough of this drama of life,’ so you go and live in the forest to meditate on Brahman.

Then finally, by practising continence and self-control, you become transformed and you enter the realm of Brahman. In that world there is a vast lake which is filled with a sweet drink. When you taste that drink you are exhilarated. There is also a banyan tree from which streams of nectar flow. You are now in a world which is not meant for the weak. Only those with strong will-power, who have practised continence and self-control over the years, can enter this world. It is the world of bliss.

The Upaniṣad says this is the third world. The first world is this earth, the second is the intermediate region, and the third is this heaven. You are close to Brahman here, and you are happy and enjoy yourself.