In plain words
Endowed with purified buddhi, restraining oneself with firmness, abandoning sound and the other sense objects, and casting aside attraction and aversion,
Word for word
Endowed with a purified buddhi, and having restrained oneself by firmness, having given up the objects beginning with sound, and having cast off attraction and aversion,
The commentators
Bhagavadgītābhāṣyam
सा इयं ज्ञानस्य परा निष्ठा उच्यते, कथं कार्या इति —
बुद्ध्या अध्यवसायलक्षणया विशुद्धया मायारहितया युक्तः सम्पन्नः, धृत्या धैर्येण आत्मानं कार्यकरणसङ्घातं नियम्य च नियमनं कृत्वा वशीकृत्य, शब्दादीन् शब्दः आदिः येषां तान् विषयान् त्यक्त्वा, सामर्थ्यात् शरीरस्थितिमात्रहेतुभूतान् केवलान् मुक्त्वा ततः अधिकान् सुखार्थान् त्यक्त्वा इत्यर्थः, शरीरस्थित्यर्थत्वेन प्राप्तेषु रागद्वेषौ व्युदस्य च परित्यज्य च ॥ ५१ ॥
In plain words
This, exactly this, is what is called the supreme establishment in knowledge. And how is it to be reached? —
'By the intellect' means by an intellect marked by firm resolve — pure, free of any trace of delusion — and 'endowed' simply means possessed of that. 'By steadiness' means by firmness, having brought the self — that is, the whole complex of body and organs — fully under control. 'Sound and the rest' means every sense-object, sound being just the first named in that list — having abandoned all of them, except, out of sheer practical necessity, whatever is merely needed to keep the body alive; everything beyond that bare minimum, everything sought only for pleasure, is what is actually given up — that is the meaning. And even toward whatever one does still receive for the sake of keeping the body alive, one casts off — truly relinquishes — all attraction and aversion.
Close to the Sanskrit
That, this — the supreme establishment of knowledge — is so called; how is it to be brought about? — thus.
'By the intellect' means by an intellect characterized by determination, pure, free from delusion — 'endowed' means possessed of that. 'By steadiness' means by firmness — having restrained the self, that is the aggregate of body and organs, having performed restraint, that is, having brought it under control. 'Sound and the rest' — sound is the first of which — those sense-objects, having abandoned them; by force of necessity, excepting only those that are merely the cause of bodily maintenance, having abandoned those beyond that which are for the sake of pleasure — this is the meaning; having cast off, and having relinquished, attraction and aversion regarding those objects that come for the purpose of bodily maintenance.