स्वे स्वे कर्मण्यभिरतः संसिद्धिं लभते नरः। स्वकर्मनिरतः सिद्धिं यथा विन्दति तच्छृणु ॥

sve sve karmaṇy abhirataḥ sansiddhiṁ labhate naraḥ sva-karma-nirataḥ siddhiṁ yathā vindati tach chhṛiṇu

Man reacheth perfection by each being intent on his own duty. Listen thou how perfection is won by him who is intent on his own duty.

In plain words: Krishna says a person reaches perfection by giving themselves fully to their own duty. He then promises to explain how this works.