चातुर्वर्ण्यं मया सृष्टं गुणकर्मविभागशः। तस्य कर्तारमपि मां विद्ध्यकर्तारमव्ययम् ॥

chātur-varṇyaṁ mayā sṛiṣhṭaṁ guṇa-karma-vibhāgaśhaḥ tasya kartāram api māṁ viddhyakartāram avyayam

The four castes were emanated by Me, by the different distribution of qualities and actions; know Me to be the author of them, though the actionless and inexhaustible.

In plain words: Krishna says he created the four groups of people in society, based on each person's nature and the work they do. He made them, yet he himself does not act and never changes.