ब्राह्मणक्षत्रियविशां शूद्राणां च परंतप। कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः ॥

brāhmaṇa-kṣhatriya-viśhāṁ śhūdrāṇāṁ cha parantapa karmāṇi pravibhaktāni svabhāva-prabhavair guṇaiḥ

Of Brâhmanas, Kshattriyas, Vaishyas and Shûdras, O Parantapa, the duties have been distributed, according to the qualities born of their own natures.

In plain words: Krishna says the duties of the four social groups of his time were divided according to the natural qualities of each. These groups were priests and teachers (Brahmanas), warriors and rulers (Kshattriyas), farmers and traders (Vaishyas), and workers who serve (Shudras).