Bhagavad Gita Chapter 18, Verse 41

ब्राह्मणक्षत्रियविशां शूद्राणां च परन्तप | कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः ||

brāhmaṇa-kṣatriya-viśāṁ śūdrāṇāṁ ca parantapa karmāṇi pravibhaktāni svabhāva-prabhavair guṇaiḥ

O conqueror of enemies, the duties of brahmanas, kshatriyas, vaishyas, and shudras are divided according to the gunas born of their nature.

Krishna introduces the teaching about the four varnas (social orders) and their duties. 'Brāhmaṇa-kṣatriya-viśāṁ śūdrāṇāṁ ca parantapa'—O conqueror of enemies (parantapa), of brahmanas, kshatriyas, vaishyas, and shudras. 'Karmāṇi pravibhaktāni svabhāva-prabhavair guṇaiḥ'—the duties (karmāṇi) are divided (pravibhaktāni) according to the gunas (guṇaiḥ) born of their nature (svabhāva-prabhavaiḥ). This is the key teaching: the four varnas (Brahmana, Kshatriya, Vaishya, Shudra) are not based on birth, but on the gunas (qualities) and nature of the individual. Their duties are divided according to their inherent qualities. This is not about caste by birth, but about understanding one's natural tendencies and performing duties accordingly. This sets up the detailed explanation of each varna's duties that follows.