In plain words
One endowed with buddhi casts off both good and evil consequences here itself. Therefore dedicate yourself to yoga; yoga is skill in action.
Word for word
One endowed with buddhi casts off here both good and evil deeds; therefore yoke yourself to yoga; yoga is skill in actions.
बुद्धियुक्तो जहातीह उभे सुकृतदुष्कृतेतस्माद् योगाय युज्यस्व योगः कर्मसु कौशलम्
Kauśalam
Kauśalam is usually translated ‘skill,’ and there is a plain picture behind the word. Every morning in the āśrama, the youngest disciples go out to gather kuśa grass for the day’s rites — the same grass this yoga is named for. Kuśa is sharp enough to cut a careless hand. So the disciple learns something more than caution: not to hold back from the grass, but to work with it so completely that the hand comes back unmarked.
That is the shape of the skill Kṛṣṇa is naming. Karmasu kauśalam is not skill kept apart from action, watching it from a safe distance. It is the hands, the body, the mind fully at work in the world, while what is underneath stays untouched by that work. The grass is sharp; the world is sharper. Remaining untouched within, even while fully engaged without — that is kauśalam.
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
समत्वबुद्धियुक्तः सन् स्वधर्ममनुतिष्ठन् यत्फलं प्राप्नोति तच्छृणु —
बुद्धियुक्तः कर्मसमत्वविषयया बुद्ध्या युक्तः बुद्धियुक्तः सः जहाति परित्यजति इह अस्मिन् लोके उभे सुकृतदुष्कृते पुण्यपापे सत्त्वशुद्धिज्ञानप्राप्तिद्वारेण यतः, तस्मात् समत्वबुद्धियोगाय युज्यस्व घटस्व । योगो हि कर्मसु कौशलम् , स्वधर्माख्येषु कर्मसु वर्तमानस्य या सिद्ध्यासिद्ध्योः समत्वबुद्धिः ईश्वरार्पितचेतस्तया तत् कौशलं कुशलभावः । तद्धि कौशलं यत् बन्धनस्वभावान्यपि कर्माणि समत्वबुद्ध्या स्वभावात् निवर्तन्ते । तस्मात्समत्वबुद्धियुक्तो भव त्वम् ॥ ५० ॥
In plain words
Hear now what comes to the one who holds that even-minded understanding and does the work that is his own to do —
'Endowed with understanding' means: joined to that understanding whose whole concern is equanimity toward action. And being joined to it, such a person lets go, right here in this life, of both good deeds and bad deeds, both merit and demerit — through the very door of a purified mind and the knowledge that follows from it. Because this is so, apply yourself, strive, toward union with that equanimous understanding. Yoga, after all, is simply skill in action — for someone carrying out their own prescribed duties, it is that equanimous understanding toward success and failure, held with a mind surrendered to Īśvara, that is the real skill, the real mastery. And this is exactly why it counts as skill: because through that equanimous understanding, actions which by their very nature would otherwise bind you simply stop binding you. So you — become one who is joined to that equanimous understanding.
Close to the Sanskrit
Being one who is joined with the understanding of evenness, performing his own dharma — which fruit he obtains, hear that —
'Endowed with understanding' [means]: endowed with the understanding whose domain is equanimity toward action; being 'endowed with understanding', he abandons, relinquishes, here, in this world, both good deeds and bad deeds, merit and demerit, through the door of the purity of mind and attainment of knowledge; because [of this,] therefore, apply yourself, strive, for union with the equanimous understanding. For yoga [is] skill in actions — for one engaged in actions called [one's] own dharma, the equanimous understanding regarding success and failure, with a mind offered up to Īśvara, by that [understanding] — that [is] skill, the state of being skillful. For that is skill, by which actions, even though bondage is their very nature, cease, by [the power of] equanimous understanding, from their [own] nature. Therefore, you, become endowed with equanimous understanding.