In plain words
A person devoted to one's own work attains perfection. Hear how one who is devoted to one's own work finds that perfection.
Word for word
Devoted each to his own duty, a person gains perfection; how one devoted to his own duty finds perfection, that hear.
The commentators
Bhagavadgītābhāṣyam
एतेषां जातिविहितानां कर्मणां सम्यगनुष्ठितानां स्वर्गप्राप्तिः फलं स्वभावतः, ‘वर्णा आश्रमाश्च स्वकर्मनिष्ठाः प्रेत्य कर्मफलमनुभूय ततः शेषेण विशिष्टदेशजातिकुलधर्मायुःश्रुतवृत्तवित्तसुखमेधसो जन्म प्रतिपद्यन्ते’(गौ. ध. २ । २ । २९) , इत्यादिस्मृतिभ्यः ; पुराणे च वर्णिनाम् आश्रमिणां च लोकफलभेदविशेषस्मरणात् । कारणान्तरात्तु इदं वक्ष्यमाणं फलम् —
स्वे स्वे यथोक्तलक्षणभेदे कर्मणि अभिरतः तत्परः संसिद्धिं स्वकर्मानुष्ठानात् अशुद्धिक्षये सति कायेन्द्रियाणां ज्ञाननिष्ठायोग्यतालक्षणां संसिद्धिं लभते प्राप्नोति नरः अधिकृतः पुरुषः ; किं स्वकर्मानुष्ठानत एव साक्षात् संसिद्धिः ? न ; कथं तर्हि ? स्वकर्मनिरतः सिद्धिं यथा येन प्रकारेण विन्दति, तत् शृणु ॥ ४५ ॥
In plain words
These duties, laid down by birth and properly carried out, naturally bear the fruit of heaven — as Smṛti itself says: 'The varṇas and āśramas, faithful to their own duties, after death, having tasted the fruit of their action, are then born again, by whatever karma remains, into a particular place, family, lineage, way of life, lifespan, learning, conduct, wealth, happiness, and intelligence' (Gautama Dharmasūtra 2.2.29) — this and similar Smṛti passages say so. And the Purāṇas, too, describe different worlds as the specific fruits reached by different varṇas and āśramas. But there is another fruit, arising from an altogether different cause, which is about to be described —
"Devoted to his own work" — wholly bent on it; each person in the work marked off for him, with the marks already described. And what he obtains is saṃsiddhi: through actually doing that work the impurity wears away, and when it is gone, body and senses become fit to stand steady in knowledge of the Self. That fitness is what "perfection" means here. The man who obtains it is the man who has a genuine claim to that work. — So does perfection come straight out of doing one's duty, and nothing more? No. How, then? Hear by what process the one absorbed in his own work comes to find it. || 45 ||
Close to the Sanskrit
Of these duties prescribed by birth, when properly performed, the attainment of heaven is the fruit, by their very nature — as it is said: 'The varṇas and āśramas, devoted to their own duties, having died, having experienced the fruit of action, thereafter, by the residue, attain a birth marked by a distinguished place, family, lineage, dharma, lifespan, learning, conduct, wealth, happiness, and intelligence' (Gautama Dharmasūtra 2.2.29) — from Smṛti texts such as this; and because in the Purāṇa, too, there is mention of specific distinctions in the fruit, namely the worlds, for those of different varṇas and āśramas. But from a different cause, this fruit about to be stated follows —
Devoted (abhirataḥ) — that is, wholly bent on it — in his own, his own action, in the division of duty whose defining marks have been stated: perfection (saṃsiddhim) — through the performance of his own duty, when impurity has been worn away, the perfection whose mark is the fitness of body and senses for settledness in knowledge — that perfection he obtains, that is, reaches, the man (naraḥ), the person who is entitled to that work. Is perfection then direct, from the performance of one's own duty alone? No. How then? Hear in what manner — by what process — one absorbed in his own work finds perfection. || 45 ||