श्रेयान्स्वधर्मो विगुणः परधर्मात्स्वनुष्ठितात्। स्वभावनियतं कर्म कुर्वन्नाप्नोति किल्बिषम् ॥

śhreyān swa-dharmo viguṇaḥ para-dharmāt sv-anuṣhṭhitāt svabhāva-niyataṁ karma kurvan nāpnoti kilbiṣham

Better is one's own duty though destitute of merits than the well-executed duty of another. He who doeth the duty laid down by his own nature incurreth not sin.

In plain words: Krishna says it is better to do your own duty imperfectly than to do someone else's duty well. Doing the work that fits your nature brings no sin.